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