#include <bits/stdc++.h>
using namespace std;
int main(void) {
  int N, M, inputs, inputc, ans = 0;
  int checkNum = 0;
  vector<int> s;
  vector<int> c;
  cin >> N >> M;
  vector<vector<int>> sc(N, vector<int>(10, 0));
  for (int i = 0; i < M; i++) {
    cin >> inputs;
    s.emplace_back(inputs);
    cin >> inputc;
    c.emplace_back(inputc);
    sc[inputs - 1][inputc]++;
  }
  for (int i = 0; i < N; i++) {
    for (int j = 0; j < 10; j++) {
      if (sc[i][j] >= 1) {
        checkNum++;
      }
    }
    if (checkNum > 1) {
      ans = -1;
      break;
    } else {
      checkNum = 0;
    }
  }
  if (ans != -1) {
    for (int i = 0; i < N; i++) {
      for (int j = 0; j < 10; j++) {
        if (sc[i][j] >= 1) {
          ans += j * pow(10, 3 - i - 1);
        }
      }
    }
  }
  if (ans / pow(10, N - 1) == 0) {
    ans = -1;
  }
  cout << ans << endl;
  return 0;
}
