#include <bits/stdc++.h>
using namespace std;
int main(void) {
  int n, m;
  cin >> n >> m;
  int a[n];
  for (int i = 0; i < n; i++) a[i] = 10;
  int f = 0;
  for (int i = 0; i < m; i++) {
    int b, c;
    cin >> b >> c;
    if (a[b - 1] > 9)
      a[b - 1] = c;
    else if (a[b - 1] != c) {
      f = 1;
      break;
    } else
      a[b - 1] = c;
  }
  for (int i = 0; i < n; i++) {
    if (a[i] > 9) a[i] = 0;
  }
  if (f == 1)
    cout << -1 << endl;
  else {
    for (int i = 0; i < n; i++) cout << a[i];
    cout << "\n";
  }
  return 0;
}
