diff --git a/Appendix D/solutions/20040.cpp b/Appendix D/solutions/20040.cpp
index 6c991660..1da8b618 100644
--- a/Appendix D/solutions/20040.cpp	
+++ b/Appendix D/solutions/20040.cpp	
@@ -1,11 +1,40 @@
-// Authored by : BaaaaaaaaaaarkingDog
+// Authored by : uhwan0723
 // Co-authored by : -
-// http://boj.kr/****************
+// http://boj.kr/f654dc349b074299a388938b0c5b35a8
 #include <bits/stdc++.h>
 using namespace std;
 
-int main(void){
+const int mn = 500005;
+int res, n, m, par[mn];
+
+int find(int x){
+  if(par[x] < 0) return x;
+  return par[x] = find(par[x]);
+}
+
+bool uni(int x, int y){
+  int u = find(x), v = find(y);
+  if(u == v) return false;
+  if(-par[u] < -par[v]) std::swap(u, v);
+  if(par[u] == par[v]) --par[u];
+  par[v] = u;
+  return true;
+}
+
+int main(){
   ios::sync_with_stdio(0);
   cin.tie(0);
-  
-}
\ No newline at end of file
+
+  cin >> n >> m;
+  fill(par, par+n+1, -1);
+  while(m--){
+    ++res;
+    int u, v;
+    cin >> u >> v;
+    if(!uni(u, v)){
+      cout << res;
+      return 0;
+    }
+  }
+  cout << 0;
+}