func_code_string: #include <bits/stdc++.h>   using namespace std;   const int N = 100009;   vector<int> edges[N];   vector<int> a;   int n, m;   int main() {     while (cin >> n >> m) {       a.clear();       for (int i = 1; i <= n; ++i) edges[i].clear(), a.push_back(i);       for (int i = 0; i < m; ++i) {         int ac, b;         cin >> ac >> b;         edges[ac].push_back(b);         edges[b].push_back(ac);       }       bool t = true;       int u, v;       for (int i = 0; i < 500; ++i) {         random_shuffle(a.begin(), a.end());         bool flg = true;         for (int j = 0; j < m && flg; ++j) {           u = a[j];           v = a[(j + 1) % n];           for (int k = 0; k < edges[u].size(); ++k)             if (edges[u][k] == v) flg = false;         }         if (flg) {           for (int j = 0; j < m; ++j)             cout << a[j] <<     << a[(j + 1) % n] << endl;           t = false;           break;         }       }       if (t) cout << -1 << endl;     }     return 0;   }   