func_code_string: #include <bits/stdc++.h>   using namespace std;   const int mod = 1e9 + 7, A = 1e5 + 2;   bool v[20][20];   long long dp[20][1 << 20];   int n, m, start = 0;   long long solve(int node, int mask) {     if (~dp[node][mask]) return dp[node][mask];     long long ret = 0;     if (v[node][start] && __builtin_popcount(mask) > 2) ret++;     for (int i = start + 1; i < n; i++) {       if (!(mask >> i & 1) && v[node][i]) ret += solve(i, mask | (1 << i));     }     return dp[node][mask] = ret;   }   int main() {     ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);     cin >> n >> m;     for (int i = 0; i < m; i++) {       int x, y;       cin >> x >> y;       x--, y--;       v[x][y] = v[y][x] = 1;     }     long long ans = 0;     memset(dp, -1, sizeof dp);     for (int i = 0; i < n; i++) start = i, ans += solve(i, 1 << i);     cout << ans / 2;     return 0;   }   