#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m;
  cin >> n >> m;
  vector<int> a(n);
  for (int i = 0; i < (n); i++) {
    a[i] = -1;
  }
  for (int i = 0; i < (m); i++) {
    int s, c;
    cin >> s >> c;
    if (a[s - 1] == -1 || a[s - 1] == c)
      a[s - 1] = c;
    else {
      cout << "-1" << endl;
      return 0;
    }
  }
  if (a[0] == -1 || a[0] == 0) {
    cout << "-1" << endl;
    return 0;
  }
  for (int i = 0; i < (n); i++) {
    if (a[i] == -1) a[i] = 0;
    cout << a[i];
  }
  cout << endl;
  return 0;
}
