#include <bits/stdc++.h>
using namespace std;
void solve(void) {
  int N, M;
  cin >> N >> M;
  int S[M], C[M];
  for (int i = 0; i < (M); i++) {
    cin >> S[i] >> C[i];
  }
  int count = -1;
  int sflag = 0;
  string A;
  for (int i = 0; i < (pow(10, N)); i++) {
    A = to_string(i);
    sflag = 0;
    for (int j = 0; j < (M); j++) {
      string D = to_string(C[j]);
      if (D[0] == A[S[j] - 1]) {
        sflag++;
      }
    }
    if (sflag == M) {
      count = i;
      break;
    }
  }
  if (N > 1 && count == 0) {
    count = -1;
  }
  cout << count << endl;
  return;
}
int main(int argc, const char* argv[]) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  solve();
  return 0;
}
