#include <bits/stdc++.h>   using namespace std;   int n, m;   set<int> s[100001];   int a[100001];   int main() {     cin >> n >> m;     for (int i = 1; i <= m; i++) {       int x, y;       cin >> x >> y;       s[x].insert(y);       s[y].insert(x);     }     srand(time(0));     for (int op = 1; op <= 200; op++) {       for (int i = 1; i <= n; i++) a[i] = i;       random_shuffle(a + 1, a + n + 1);       a[n + 1] = a[1];       int i;       for (i = 1; i <= m; i++) {         if (s[a[i]].count(a[i + 1])) break;       }       if (i == m + 1) {         for (i = 1; i <= m; i++) cout << a[i] <<     << a[i + 1] << endl;         return 0;       }     }     cout << -1;     return 0;   }   