diff --git a/.gitignore b/.gitignore
index a1c2a23..0b4f76c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,5 @@
+ProfCalculator/.classpath
+.project
 # Compiled class file
 *.class
 
@@ -21,3 +23,101 @@
 
 # virtual machine crash logs, see http://www.java.com/en/download/help/error_hotspot.xml
 hs_err_pid*
+
+
+# Created by https://www.toptal.com/developers/gitignore/api/java,eclipse
+# Edit at https://www.toptal.com/developers/gitignore?templates=java,eclipse
+
+### Eclipse ###
+.metadata
+bin/
+tmp/
+*.tmp
+*.bak
+*.swp
+*~.nib
+local.properties
+.settings/
+.loadpath
+.recommenders
+
+# External tool builders
+.externalToolBuilders/
+
+# Locally stored "Eclipse launch configurations"
+*.launch
+
+# PyDev specific (Python IDE for Eclipse)
+*.pydevproject
+
+# CDT-specific (C/C++ Development Tooling)
+.cproject
+
+# CDT- autotools
+.autotools
+
+# Java annotation processor (APT)
+.factorypath
+
+# PDT-specific (PHP Development Tools)
+.buildpath
+
+# sbteclipse plugin
+.target
+
+# Tern plugin
+.tern-project
+
+# TeXlipse plugin
+.texlipse
+
+# STS (Spring Tool Suite)
+.springBeans
+
+# Code Recommenders
+.recommenders/
+
+# Annotation Processing
+.apt_generated/
+.apt_generated_test/
+
+# Scala IDE specific (Scala & Java development for Eclipse)
+.cache-main
+.scala_dependencies
+.worksheet
+
+# Uncomment this line if you wish to ignore the project description file.
+# Typically, this file would be tracked if it contains build/dependency configurations:
+#.project
+
+### Eclipse Patch ###
+# Spring Boot Tooling
+.sts4-cache/
+
+### Java ###
+# Compiled class file
+*.class
+
+# Log file
+*.log
+
+# BlueJ files
+*.ctxt
+
+# Mobile Tools for Java (J2ME)
+.mtj.tmp/
+
+# Package Files #
+*.jar
+*.war
+*.nar
+*.ear
+*.zip
+*.tar.gz
+*.rar
+
+# virtual machine crash logs, see http://www.java.com/en/download/help/error_hotspot.xml
+hs_err_pid*
+
+# End of https://www.toptal.com/developers/gitignore/api/java,eclipse
+
diff --git a/ProfCalculator/.classpath b/ProfCalculator/.classpath
index 2b1ff5c..19eb77e 100644
--- a/ProfCalculator/.classpath
+++ b/ProfCalculator/.classpath
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <classpath>
-	<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/JavaSE-13">
+	<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/java-11-openjdk">
 		<attributes>
 			<attribute name="module" value="true"/>
 		</attributes>
@@ -8,5 +8,6 @@
 	<classpathentry kind="con" path="org.eclipse.jdt.USER_LIBRARY/JavaFx"/>
 	<classpathentry kind="con" path="org.eclipse.jdt.USER_LIBRARY/JavaFX"/>
 	<classpathentry kind="src" path="src"/>
+	<classpathentry kind="con" path="org.eclipse.jdt.junit.JUNIT_CONTAINER/5"/>
 	<classpathentry kind="output" path="bin"/>
 </classpath>
diff --git a/ProfCalculator/.settings/org.eclipse.jdt.core.prefs b/ProfCalculator/.settings/org.eclipse.jdt.core.prefs
index ef28d2b..cd8d089 100644
--- a/ProfCalculator/.settings/org.eclipse.jdt.core.prefs
+++ b/ProfCalculator/.settings/org.eclipse.jdt.core.prefs
@@ -1,8 +1,9 @@
 eclipse.preferences.version=1
 org.eclipse.jdt.core.compiler.codegen.inlineJsrBytecode=enabled
-org.eclipse.jdt.core.compiler.codegen.targetPlatform=13
+org.eclipse.jdt.core.compiler.codegen.methodParameters=do not generate
+org.eclipse.jdt.core.compiler.codegen.targetPlatform=11
 org.eclipse.jdt.core.compiler.codegen.unusedLocal=preserve
-org.eclipse.jdt.core.compiler.compliance=13
+org.eclipse.jdt.core.compiler.compliance=11
 org.eclipse.jdt.core.compiler.debug.lineNumber=generate
 org.eclipse.jdt.core.compiler.debug.localVariable=generate
 org.eclipse.jdt.core.compiler.debug.sourceFile=generate
@@ -10,5 +11,5 @@ org.eclipse.jdt.core.compiler.problem.assertIdentifier=error
 org.eclipse.jdt.core.compiler.problem.enablePreviewFeatures=disabled
 org.eclipse.jdt.core.compiler.problem.enumIdentifier=error
 org.eclipse.jdt.core.compiler.problem.reportPreviewFeatures=warning
-org.eclipse.jdt.core.compiler.release=enabled
-org.eclipse.jdt.core.compiler.source=13
+org.eclipse.jdt.core.compiler.release=disabled
+org.eclipse.jdt.core.compiler.source=11
diff --git a/ProfCalculator/src/de/uulm/sp/swt/profcalculator/ProfCalculator.java b/ProfCalculator/src/de/uulm/sp/swt/profcalculator/ProfCalculator.java
index 20f6660..65819b3 100644
--- a/ProfCalculator/src/de/uulm/sp/swt/profcalculator/ProfCalculator.java
+++ b/ProfCalculator/src/de/uulm/sp/swt/profcalculator/ProfCalculator.java
@@ -5,6 +5,7 @@
 import de.uulm.sp.swt.profcalculator.expressions.Expression;
 import de.uulm.sp.swt.profcalculator.expressions.Multiplication;
 import de.uulm.sp.swt.profcalculator.expressions.NecessaryBrackets;
+import de.uulm.sp.swt.profcalculator.expressions.Sub;
 import de.uulm.sp.swt.profcalculator.expressions.Value;
 import de.uulm.sp.swt.profcalculator.gui.BlueFontGUIFactory;
 import de.uulm.sp.swt.profcalculator.gui.GUIFactory;
@@ -20,10 +21,10 @@
 import javafx.scene.paint.Color;
 import javafx.stage.Stage;
 
-public class ProfCalculator	extends Application implements EventHandler<ActionEvent> {
+public class ProfCalculator extends Application implements EventHandler<ActionEvent> {
 
 	private Expression expression = new CounterValue(this);
-	
+
 	private GUIFactory guiFactory = new BlueFontGUIFactory();
 
 	private Label errorLabel = guiFactory.createLabel();
@@ -31,6 +32,7 @@ public class ProfCalculator	extends Application implements EventHandler<ActionEv
 	private TextField inputField = new TextField();
 
 	private Button additionButton = guiFactory.createButton("+");
+	private Button subButton = guiFactory.createButton("-");
 	private Button multiplicationButton = guiFactory.createButton("*");
 
 	private Label resultLabel = guiFactory.createLabel();
@@ -40,13 +42,14 @@ public void start(Stage stage) throws Exception {
 		stage.setTitle("Professorial Calculator");
 		errorLabel.setTextFill(Color.web("#AA0000"));
 
-		VBox layout = new VBox(10, errorLabel, inputField, additionButton, multiplicationButton, resultLabel);
+		VBox layout = new VBox(10, errorLabel, inputField, additionButton, subButton, multiplicationButton, resultLabel);
 		layout.setPadding(new Insets(20, 80, 20, 80));
 		Scene scene = new Scene(layout);
 
 		stage.setScene(scene);
 		stage.show();
 		additionButton.setOnAction(this);
+		subButton.setOnAction(this);
 		multiplicationButton.setOnAction(this);
 		updateGUI();
 	}
@@ -58,8 +61,10 @@ public void handle(ActionEvent event) {
 			if (event.getSource() == additionButton) {
 				expression = new Addition(expression, new Value(newValue));
 				Logger.getLogger().log("+ " + newValue);
-			}
-			else if (event.getSource() == multiplicationButton) {
+			} else if (event.getSource() == subButton) {
+				expression = new Sub(expression, new Value(newValue));
+				Logger.getLogger().log("- " + newValue);
+			} else if (event.getSource() == multiplicationButton) {
 				expression = new Multiplication(expression, new Value(newValue));
 				Logger.getLogger().log("* " + newValue);
 			}
diff --git a/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/Addition.java b/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/Addition.java
index 290f903..6b3e437 100644
--- a/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/Addition.java
+++ b/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/Addition.java
@@ -11,6 +11,9 @@ public Addition(Expression left, Expression right) {
 	}
 
 	public String toString(Expression parent) {
+		if (right.evaluate() < 0) {
+			return left.toString(this) + " + (" + right.toString(this) + ")";
+		}
 		return left.toString(this) + " + " + right.toString(this);
 	}
 
diff --git a/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/CounterValue.java b/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/CounterValue.java
index 7262a41..c9424da 100644
--- a/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/CounterValue.java
+++ b/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/CounterValue.java
@@ -15,7 +15,7 @@ public CounterValue(ProfCalculator calc) {
 
 	@Override
 	public void run() {
-		while (value < 1) {
+		while (value < 0) {
 			try {
 				Thread.sleep(1000);
 			} catch (InterruptedException e) {
diff --git a/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/Multiplication.java b/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/Multiplication.java
index ba5f46e..becbab9 100644
--- a/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/Multiplication.java
+++ b/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/Multiplication.java
@@ -11,6 +11,9 @@ public Multiplication(Expression left, Expression right) {
 	}
 
 	public String toString(Expression parent) {
+		if (right.evaluate() < 0) {
+			return left.toString(this) + " * (" + right.toString(this) + ")";
+		}
 		return left.toString(this) + " * " + right.toString(parent);
 	}
 
diff --git a/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/NecessaryBrackets.java b/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/NecessaryBrackets.java
index d0eeea8..5ebd1c2 100644
--- a/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/NecessaryBrackets.java
+++ b/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/NecessaryBrackets.java
@@ -7,11 +7,11 @@ public class NecessaryBrackets extends Expression {
 	public NecessaryBrackets(Expression expression) {
 		this.expression = expression;
 	}
-	
+
 	@Override
 	public String toString(Expression parent) {
 		String childString = expression.toString(parent);
-		if (expression instanceof Multiplication && parent instanceof Addition) {
+		if (parent instanceof Multiplication && (expression instanceof Addition || expression instanceof Sub)) {
 			childString = "(" + childString + ")";
 		}
 		return childString;
diff --git a/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/Sub.java b/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/Sub.java
new file mode 100644
index 0000000..34a6b5a
--- /dev/null
+++ b/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/Sub.java
@@ -0,0 +1,23 @@
+package de.uulm.sp.swt.profcalculator.expressions;
+
+public class Sub extends Expression {
+
+	public Expression left;
+	public Expression right;
+
+	public Sub(Expression left, Expression right) {
+		this.left = left;
+		this.right = right;
+	}
+
+	public String toString(Expression parent) {
+		if (right.evaluate() < 0) {
+			return left.toString(this) + " - (" + right.toString(this) + ")";
+		}
+		return left.toString(this) + " - " + right.toString(this);
+	}
+
+	public int evaluate() {
+		return left.evaluate() - right.evaluate();
+	}
+}
\ No newline at end of file
diff --git a/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/test/TNecessaryBracketsWhiteBox.java b/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/test/TNecessaryBracketsWhiteBox.java
new file mode 100644
index 0000000..2425feb
--- /dev/null
+++ b/ProfCalculator/src/de/uulm/sp/swt/profcalculator/expressions/test/TNecessaryBracketsWhiteBox.java
@@ -0,0 +1,79 @@
+package de.uulm.sp.swt.profcalculator.expressions.test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+import de.uulm.sp.swt.profcalculator.expressions.Addition;
+import de.uulm.sp.swt.profcalculator.expressions.Expression;
+import de.uulm.sp.swt.profcalculator.expressions.Multiplication;
+import de.uulm.sp.swt.profcalculator.expressions.NecessaryBrackets;
+import de.uulm.sp.swt.profcalculator.expressions.Sub;
+import de.uulm.sp.swt.profcalculator.expressions.Value;
+
+class TNecessaryBracketsWhiteBox {
+
+	@Test
+	void testAdditionWithinMultiplication() {
+		Expression child = new Addition(new Value(1), new Value(2));
+		Expression brackets = new NecessaryBrackets(child);
+		Expression parent = new Multiplication(brackets, new Value(3));
+		assertEquals(parent.toString(), "(1 + 2) * 3");
+	}
+
+	@Test
+	void testMultiplicationWithinAddition() {
+		Expression child = new Multiplication(new Value(1), new Value(2));
+		Expression brackets = new NecessaryBrackets(child);
+		Expression parent = new Addition(brackets, new Value(3));
+		assertEquals(parent.toString(), "1 * 2 + 3");
+	}
+
+	@Test
+	void testMultiplicationWithinMultiplication() {
+		Expression child = new Multiplication(new Value(1), new Value(2));
+		Expression brackets = new NecessaryBrackets(child);
+		Expression parent = new Multiplication(brackets, new Value(3));
+		assertEquals(parent.toString(), "1 * 2 * 3");
+	}
+
+	@Test
+	void testSubtractionWithinMultiplication() {
+		Expression child = new Sub(new Value(1), new Value(2));
+		Expression brackets = new NecessaryBrackets(child);
+		Expression parent = new Multiplication(brackets, new Value(3));
+		assertEquals(parent.toString(), "(1 - 2) * 3");
+	}
+
+	@Test
+	void testMultiplicationWithinSubtraction() {
+		Expression child = new Multiplication(new Value(1), new Value(2));
+		Expression brackets = new NecessaryBrackets(child);
+		Expression parent = new Sub(brackets, new Value(3));
+		assertEquals(parent.toString(), "1 * 2 - 3");
+	}
+	
+	@Test
+	void testMultiplicationWithinSubtractionNegativeNumber() {
+		Expression child = new Multiplication(new Value(1), new Value(2));
+		Expression brackets = new NecessaryBrackets(child);
+		Expression parent = new Sub(brackets, new Value(-3));
+		assertEquals(parent.toString(), "1 * 2 - (-3)");
+	}
+	
+	@Test
+	void testSubtractionWithinMultiplicationWithNegativeNumber() {
+		Expression child = new Sub(new Value(1), new Value(2));
+		Expression brackets = new NecessaryBrackets(child);
+		Expression parent = new Multiplication(brackets, new Value(-3));
+		assertEquals(parent.toString(), "(1 - 2) * (-3)");
+	}
+	
+	@Test
+	void testMultiplicationWithinAdditionWithNegativeNumber() {
+		Expression child = new Multiplication(new Value(1), new Value(2));
+		Expression brackets = new NecessaryBrackets(child);
+		Expression parent = new Addition(brackets, new Value(-3));
+		assertEquals(parent.toString(), "1 * 2 + (-3)");
+	}
+}