#include <bits/stdc++.h>
using namespace std;
int main() {
  int N, M, T, X;
  cin >> N >> M;
  vector<vector<int>> vec_a(M, vector<int>(2));
  vector<int> vec_b(N, 0);
  T = 0;
  for (int i = 0; i < M; i++) {
    for (int j = 0; j < 2; j++) {
      cin >> vec_a[i][j];
    }
  }
  for (int i = 0; i < M - 1; i++) {
    for (int j = i + 1; j < M; j++) {
      if (vec_a[i][0] == vec_a[j][0] && vec_a[i][1] != vec_a[j][1]) {
        T = 1;
      }
    }
  }
  for (int i = 0; i < M; i++) {
    X = vec_a[i][0] - 1;
    vec_b[X] = vec_a[i][1];
  }
  if (N != 1 && vec_b[0] == 0) {
    T = 1;
  }
  if (T == 0) {
    for (int i = 0; i < M; i++) {
      cout << vec_b[i];
    }
  }
  if (T == 1) {
    cout << "-1" << endl;
  }
}
