#include <bits/stdc++.h>
using namespace std;
int main(void) {
  int N, M;
  cin >> N >> M;
  string ans = "1";
  if (N >= 2) {
    for (int i = 0; i < N - 1; i++) ans += "0";
    int s;
    char c;
    map<int, char> mp;
    for (int i = 0; i < M; i++) {
      cin >> s >> c;
      s--;
      if (s == 0 && c == '0') {
        ans = "-1";
        break;
      } else if (mp[s] == 0 || mp[s] == c) {
        ans[s] = c;
        mp[s] = c;
      } else {
        ans = "-1";
        break;
      }
    }
  } else {
    int s;
    char c;
    map<int, char> mp;
    for (int i = 0; i < M; i++) {
      cin >> s >> c;
      s--;
      if (mp[s] == 0 || mp[s] == c) {
        ans[s] = c;
        mp[s] = c;
      } else {
        ans = "-1";
        break;
      }
    }
  }
  cout << ans << endl;
}
