#include <bits/stdc++.h>
using namespace std;
int main(void) {
  int N, M;
  cin >> N >> M;
  if (M == 0) {
    if (N == 1)
      cout << 0 << endl;
    else
      cout << pow(10, N - 1) << endl;
  } else {
    vector<set<int>> vec(11);
    for (int i = 0; i < M; ++i) {
      int a, b;
      cin >> a >> b;
      vec[a].insert(b);
    }
    for (int i = 1; i <= M; ++i) {
      if (vec[i].size() > 1) {
        cout << "-1\n";
        return 0;
      }
    }
    int num = 0;
    for (int i = 1; i <= M; ++i) {
      if (vec[i].size() != 0) num += *vec[i].begin();
      num *= 10;
    }
    if (num == 0 && N != 1) {
      cout << "-1\n";
      return 0;
    }
    cout << num / 10 << "\n";
  }
}
