#include <bits/stdc++.h>
using namespace std;
int main() {
  int N, M;
  cin >> N >> M;
  vector<int> s_al(N, -1);
  if (N == 1) {
    int s, c;
    for (int i = 0; i < M; i++) {
      cin >> s >> c;
      if (s_al[0] != c && s_al[0] != -1) {
        cout << "-1" << endl;
        return 0;
      } else {
        s_al[02] = c;
      }
    }
    if (s_al[0] == -1) s_al[0] = 0;
    cout << s_al[0] << endl;
  } else {
    int s, c;
    for (int i = 0; i < M; i++) {
      cin >> s >> c;
      if (s_al[s - 1] != -1 && s_al[s - 1] != c) {
        cout << "-1" << endl;
        return 0;
      } else if (s == 0 && c == 0) {
        cout << "-1" << endl;
        return 0;
      } else {
        s_al[s - 1] = c;
      }
    }
    for (int i = 0; i < N; i++) {
      if (s_al[i] == -1 && i == 0) s_al[i] = 1;
      if (s_al[i] == -1 && i != 0) s_al[i] = 0;
    }
    reverse(s_al.begin(), s_al.end());
    int ans = 0;
    for (int i = 0; i < N; i++) {
      ans += s_al[i] * pow(10, i);
    }
    cout << ans << endl;
  }
}
