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