E. Cyclic Components (DFS)（Codeforces Round #479 (Div. 3)）

 #include <bits/stdc++.h>
 using namespace std;

 const int maxn = *1e5+;
 vector<int>p[maxn];
 vector<int>c;
 bool check[maxn];

 void dfs(int x) {
     check[x] = true;
     c.push_back(x);
     for (int i = ; i < p[x].size(); i++) {
         int y = p[x][i];
         if (!check[y]) {
             dfs(y);
         }
     }
 }

 int main() {
     ios_base::sync_with_stdio(false);
     cin.tie();

     int n, m;
     cin >> n >> m;
     for (int i = ; i < m; i++) {
         int u, v;
         cin >> u >> v;
         p[u].push_back(v);
         p[v].push_back(u);
     }
     int ans = ;
     for (int i = ; i <= n; i++) {
         if (!check[i]) {
             c.clear();
             dfs(i);
             bool flag = true;
             for (int k = ; k < c.size(); k++) {
                 if (p[c[k]].size() != ) {
                     flag = false;
                     break;
                 }
             }
             if(flag)
                 ans++;
         }
     }
     cout << ans << endl;
     return ;
 }