diff --git a/network.py b/network.py
index 40a3e36..04b15bc 100644
--- a/network.py
+++ b/network.py
@@ -58,8 +58,13 @@ def evaluate(self, test_data):
     def cost_derivative(self, output_activations, y):
         return (output_activations-y)
 
+
 def sigmoid(z):
     return 1.0/(1.0+np.exp(-z))
 
 def sigmoid_prime(z):
     return sigmoid(z)*(1-sigmoid(z))
+
+def cross_entropy(a, y):    
+    return np.sum(np.nan_to_num(-y*np.log(a)-(1-y)*np.log(1-a)))
+