#include <bits/stdc++.h>
using namespace std;
int main() {
  int N, M;
  cin >> N >> M;
  vector<int> S(M);
  vector<char> C(M);
  for (int i = 0; i < M; i++) {
    cin >> S.at(i) >> C.at(i);
  }
  bool flag = false;
  for (int i = 0; i < M; i++) {
    for (int j = 0; j < M; j++) {
      if (S.at(i) == S.at(j) && C.at(i) != C.at(j))
        ;
      flag = true;
    }
  }
  vector<char> number(N, '0');
  for (int i = 0; i < M; i++) {
    number.at(S.at(i) - 1) = C.at(i);
  }
  if ((N == 3 && number.at(0) == '0') ||
      (N == 2 && number.at(0) == '0' && number.at(1) == 0) || flag) {
    cout << -1 << endl;
  } else {
    for (int i = 0; i < N; i++) {
      cout << number.at(i);
    }
  }
  return 0;
}
