#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m;
  cin >> n >> m;
  vector<int> s(m), c(m);
  for (int i = 0; i < (m); ++i) cin >> s[i] >> c[i];
  for (int i = 0; i < (m); ++i) {
    for (int j = 0; j < (m); ++j) {
      if (s[i] == s[j] && c[i] != c[j]) {
        cout << -1 << endl;
        return 0;
      }
    }
  }
  if (n >= 2) {
    for (int i = 0; i < (m); ++i) {
      if (s[i] == 1 && c[i] == 0) {
        cout << -1 << endl;
        return 0;
      }
    }
  }
  int ans = 0;
  bool o = false, p, q;
  for (int i = 0; i < (m); ++i) {
    if ((ans / (int)pow(10, n - s[i])) % 10 == 0)
      ans += pow(10, n - s[i]) * c[i];
  }
  cout << ans << endl;
  return 0;
}
