#include <bits/stdc++.h>
using namespace std;
int main() {
  int keta, num;
  int tmp1, tmp2;
  cin >> keta >> num;
  map<int, int> m;
  vector<int> vec(10, 0);
  for (int i = 0; i < num; i++) {
    cin >> tmp1 >> tmp2;
    if (vec.at(tmp1) != 0 && m[tmp1] != tmp2) {
      cout << -1;
      return 0;
    }
    vec.at(tmp1) += 1;
    m[tmp1] = tmp2;
  }
  for (int i = 1; i <= keta; i++) {
    if (i == 1 && m[i] == 0) {
      cout << 1;
    } else {
      cout << m[i];
    }
  }
}
