#include <bits/stdc++.h>
using namespace std;
int main() {
  int N, M;
  cin >> N >> M;
  vector<int> s(M);
  vector<int> c(M);
  vector<int> ans(M);
  int anser = 0;
  for (int i = 0; i < M; i++) {
    cin >> s.at(i) >> c.at(i);
    if (ans.at(s.at(i) - 1) == 0) {
      ans.at(s.at(i) - 1) = c.at(i);
    }
  }
  bool can = true;
  for (int i = 0; i < M - 1; i++) {
    for (int j = i + 1; j < M; j++) {
      if (s.at(i) == s.at(j) && c.at(i) != c.at(j)) {
        can = false;
        break;
      }
    }
    if (!can) break;
  }
  if (!can || (N >= 2 && ans.at(0) == 0))
    cout << -1;
  else {
    for (int i = 0; i < M; i++) {
      anser += pow(10, N - 1 - i) * ans.at(i);
    }
    cout << anser;
  }
}
