#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, i, x[3] = {10, 10, 10}, c, s;
  cin >> n >> m;
  if (n == 1) {
    for (i = 0; i < m; i++) {
      cin >> s >> c;
      if (x[0] != 10 && x[0] != c) {
        cout << "-1";
        return 0;
      } else
        x[0] = c;
    }
    cout << x[0];
    return 0;
  }
  if (n == 2) {
    for (i = 0; i < m; i++) {
      cin >> s >> c;
      if (x[s - 1] != 10 && x[s - 1] != c) {
        cout << "-1";
        return 0;
      }
      if (s == 1 && c == 0) {
        cout << "-1";
        return 0;
      } else
        x[s - 1] = c;
    }
    if (x[0] == 10) x[0] = 1;
    if (x[1] == 10) x[1] = 0;
    cout << x[0] << x[1];
  }
  if (n == 3) {
    for (i = 0; i < m; i++) {
      cin >> s >> c;
      if (x[s - 1] != 10 && x[s - 1] != c) {
        cout << "-1";
        return 0;
      }
      if (s == 1 && c == 0) {
        cout << "-1";
        return 0;
      } else
        x[s - 1] = c;
    }
    if (x[0] == 10) x[0] = 1;
    if (x[1] == 10) x[1] = 0;
    if (x[2] == 10) x[2] = 0;
    cout << x[0] << x[1] << x[2];
  }
}
