#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 5;
int n, m, a[10];
int main() {
  cin >> n >> m;
  memset(a, -1, sizeof a);
  int x, y, f = 1;
  while (m--) {
    cin >> x >> y;
    if (a[x] != -1 && a[x] != y)
      f = 0;
    else
      a[x] = y;
  }
  if (f == 0 || (n > 1 && a[1] == 0))
    puts("-1");
  else {
    for (int i = 1; i <= n; i++) cout << max(0, a[i]);
    cout << endl;
  }
}
