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

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

const int maxn = 2*1e5+5;
vector<int>p[maxn];
vector<int>c;
bool check[maxn];


void dfs(int x) {
    check[x] = true;
    c.push_back(x);
    for (int i = 0; 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(0);

    int n, m;
    cin >> n >> m;
    for (int i = 0; i < m; i++) {
        int u, v;
        cin >> u >> v;
        p[u].push_back(v);
        p[v].push_back(u);
    }
    int ans = 0;
    for (int i = 1; i <= n; i++) {
        if (!check[i]) {
            c.clear();
            dfs(i);
            bool flag = true;
            for (int k = 0; k < c.size(); k++) {
                if (p[c[k]].size() != 2) {
                    flag = false;
                    break;
                }
            }
            if(flag)
                ans++;
        }
    }
    cout << ans << endl;
    return 0;
}