#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 < 1000; i++) {
    string t = to_string(i);
    t += "  ";
    bool f = true;
    for (int j = 0; j < m; j++) {
      if (t[s[j] - 1] != ('0' + c[j])) {
        f = false;
        break;
      }
    }
    if (f) {
      cout << i << endl;
      return 0;
    }
  }
  cout << -1 << endl;
}
