#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1000000007;
int main() {
  int n, m, s, c;
  set<int> a;
  bool ans = true;
  cin >> n >> m;
  vector<int> num(n, -1);
  for (int i = 0; i < (int)(m); i++) {
    cin >> s >> c;
    if (num.at(s - 1) != -1 && num.at(s - 1) != c)
      ans = false;
    else {
      num.at(s - 1) = c;
    }
  }
  if (num.at(0) == 0 && n != 1) ans = false;
  if (ans) {
    for (int i = 0; i < (int)(n); i++) {
      if (num.at(i) != -1)
        cout << num.at(i);
      else
        cout << 1;
    }
    cout << endl;
  } else
    cout << -1 << endl;
}
