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