#include <bits/stdc++.h>
using namespace std;
int main() {
  ostringstream oss;
  int n, m, flag = 0, sum = 0;
  cin >> n >> m;
  vector<int> vec(n);
  for (int i = 0; i < n; i++) {
    vec.at(i) = -1;
  }
  for (int i = 0; i < m; i++) {
    int s, c;
    cin >> s >> c;
    if (vec.at(s - 1) == -1 || vec.at(s - 1) == c) {
      vec.at(s - 1) = c;
    } else {
      flag = -1;
      cout << -1;
      break;
    }
  }
  for (int i = 0; i < n; i++) {
    if (vec.at(i) > 0) {
      sum += vec.at(i) * pow(10, n - i - 1);
    }
  }
  if (flag == 0) {
    cout << sum;
  }
}
