#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m;
  cin >> n >> m;
  int t[m], c[m];
  for (int i = 0; i < m; i++) {
    cin >> t[i] >> c[i];
  }
  int ret = -1;
  for (int i = 0; i < 1000; i++) {
    string s = to_string(i);
    if (s.length() != n) continue;
    bool val = true;
    for (int j = 0; j < m; j++) {
      val &= s[t[j] - 1] - '0' == c[j];
      if (val) {
        ret = i;
        break;
      }
    }
  }
  cout << ret << endl;
  return 0;
}
