#include <bits/stdc++.h>
using namespace std;
int main() {
  int N, M;
  cin >> N >> M;
  vector<pair<int, int>> sc(M);
  for (int i = 0; i < M; ++i) {
    cin >> sc[i].first >> sc[i].second;
  }
  vector<pair<int, bool>> number(N);
  for (int i = 0; i < N; ++i) {
    number[i].first = 0;
    number[i].second = true;
  }
  for (int i = 0; i < M; ++i) {
    if (number[sc[i].first - 1].second == true) {
      number[sc[i].first - 1].first = sc[i].second;
      number[sc[i].first - 1].second = false;
    } else if (number[sc[i].first - 1].first == sc[i].second) {
    } else {
      cout << "-1" << endl;
      return 0;
    }
  }
  if (number[0].first == 0 && N > 1) {
    cout << "-1" << endl;
    return 0;
  }
  for (int i = 0; i < N; ++i) {
    cout << number[i].first;
  }
  cout << endl;
  return 0;
}
