#include <bits/stdc++.h>
using namespace std;
int main(void) {
  long long n, m;
  cin >> n >> m;
  vector<pair<long long, long long>> check;
  for (long long i = 0; i <= 1000; i++) {
    check.emplace_back(i, 0);
  }
  for (long long i = 0; i < m; i++) {
    long long s, c;
    cin >> s >> c;
    for (long long i = 1; i < 1000; i++) {
      string str = to_string(i);
      string cc = to_string(c);
      if (str[s - 1] != cc[0] || str.length() != n) check[i].second = 1;
    }
  }
  for (long long i = 1; i < 1000; i++) {
    if (check[i].second == 0) {
      cout << i << endl;
      break;
    }
    if (i == 999) cout << -1 << endl;
  }
  return 0;
}
