func_code_string: #include <bits/stdc++.h>   using namespace std;   const int mo = 1000000007;   const int inv2 = (mo + 1) / 2;   const int inv4 = (mo + 1) / 4;   const int N = 500005;   int dep[N], fa[N], deg[N];   int sz[N], p[N], cnt[N], n, m, nd;   vector<int> e[N], vec[N];   int power(int x, int y) {     int s = 1;     for (; y; y /= 2, x = 1ll * x * x % mo)       if (y & 1) s = 1ll * s * x % mo;     return s;   }   void dfs(int x) {     dep[x] = dep[fa[x]] + 1;     for (auto i : e[x])       if (i != fa[x])         if (!dep[i]) {           fa[i] = x;           dfs(i);         } else if (dep[x] > dep[i]) {           ++nd;           for (int y = x;; y = fa[y]) {             vec[nd].push_back(y);             if (y == i) break;           }           sz[nd] = vec[nd].size();           p[nd] = power(inv2, sz[nd]);         }   }   int main() {     scanf( %d%d , &n, &m);     for (int i = (int)(1); i <= (int)(m); i++) {       int x, y;       scanf( %d%d , &x, &y);       e[x].push_back(y);       e[y].push_back(x);     }     for (int i = (int)(1); i <= (int)(n); i++) deg[i] = e[i].size();     dfs(1);     int E1 = 0, E2 = 0, E3 = 0;     E1 = 1ll * n * inv2 % mo;     E2 = 1ll * m * inv4 % mo;     for (int i = (int)(1); i <= (int)(nd); i++) E3 = (E3 + p[i]) % mo;     int E = (E1 + mo - E2 + 1ll * E3) % mo;     int E11 = 0, E12 = 0, E13 = 0, E22 = 0, E23 = 0, E33 = 0;     E11 = (1ll * n * (n - 1) % mo * inv4 % mo + E1) % mo;     for (int i = (int)(1); i <= (int)(n); i++)       E12 = (E12 + 1ll * (m - deg[i]) * inv4 % mo * inv2 + 1ll * deg[i] * inv4) %             mo;     for (int i = (int)(1); i <= (int)(nd); i++)       E13 =           (E13 + 1ll * p[i] * (1ll * inv2 * (n - sz[i]) % mo + sz[i]) % mo) % mo;     E22 = (1ll * m * (m - 1) % mo * inv4 % mo * inv4 % mo + E2) % mo;     for (int i = (int)(1); i <= (int)(n); i++)       E22 = (E22 + 1ll * deg[i] * (deg[i] - 1) % mo * inv4 % mo * inv4) % mo;     for (int i = (int)(1); i <= (int)(nd); i++)       E23 = (E23 + 1ll * p[i] * (1ll * inv4 * (m - sz[i]) % mo + sz[i])) % mo;     for (int i = (int)(1); i <= (int)(nd); i++)       for (auto j : vec[i])         E23 = (E23 + 1ll * p[i] * inv4 % mo * (deg[j] - 2) % mo) % mo;     for (int i = (int)(1); i <= (int)(nd); i++)       E33 = (E33 + 1ll * p[i] * (E3 + mo - p[i] + 1)) % mo;     for (int i = (int)(1); i <= (int)(nd); i++)       for (auto j : vec[i]) cnt[j] = (cnt[j] + p[i]) % mo;     for (int i = (int)(1); i <= (int)(nd); i++)       for (auto j : vec[i]) E33 = (E33 + 1ll * p[i] * (cnt[j] + mo - p[i])) % mo;     int EF =         (1ll * E11 + E22 + E33 + 2 * E13 + 4ll * mo - 2 * E12 - 2 * E23) % mo;     printf( %lld n , (EF + mo - 1ll * E * E % mo) % mo);   }   