#include <bits/stdc++.h>
using namespace std;
int main(int argc, char const *argv[]) {
  int N, M;
  cin >> N >> M;
  string num;
  for (int i = 0; i < N; i++) {
    num.push_back('a');
  }
  int s;
  char c;
  for (int i = 0; i < M; i++) {
    cin >> s >> c;
    if (num[s - 1] == 'a') {
      num[s - 1] = c;
    } else {
      if (c != num[s - 1]) {
        cout << -1 << endl;
        return 0;
      }
    }
  }
  if (num[0] == 'a') num[0] = '1';
  if (num[0] == '0') {
    cout << -1 << endl;
    return 0;
  }
  for (int i = 1; i < num.size(); i++) {
    if (num[i] == 'a') num[i] = '0';
  }
  int ans = stoi(num);
  cout << ans << endl;
  return 0;
}
