#include <bits/stdc++.h>
using namespace std;
int solver() {
  int n, m;
  cin >> n >> m;
  int num[n];
  for (int i = 0; i < n; i++) {
    num[i] = -1;
  }
  if (n == 1 and m == 0) return 0;
  if (n != 1 and m == 0) return -1;
  int minDigit = 1000000;
  for (int i = 0; i < m; i++) {
    int s, c;
    cin >> s >> c;
    if (num[s - 1] != c and num[s - 1] != -1) {
      return -1;
    }
    if (minDigit > s) minDigit = s;
    num[s - 1] = c;
  }
  if (num[minDigit - 1] == 0) {
    if (minDigit == 1 and n != 1) return -1;
    num[0] = 1;
  }
  int ans = 0;
  for (int i = 0; i < n; i++) {
    if (num[n - 1 - i] > 0) {
      ans += num[n - 1 - i] * pow(10, i);
    }
  }
  return ans;
}
int main() {
  int ans = solver();
  cout << ans << "\n";
  return 0;
}
