#include <bits/stdc++.h>
using namespace std;
int main() {
  int a[5];
  a[0] = -1;
  a[1] = -1;
  a[2] = -1;
  int n, m;
  cin >> n >> m;
  int s, c;
  for (int i = 0; i < m; i++) {
    cin >> s >> c;
    if (a[n - s] == -1) {
      a[n - s] = c;
    } else if (a[n - s] != c) {
      cout << -1;
      return 0;
    }
  }
  if (a[0] == 0) {
    cout << -1;
    return 0;
  }
  cout << max(a[0], 1) + max(a[1], 0) * 10 + max(a[2], 0) * 100;
}
