#include <bits/stdc++.h>
using namespace std;
int main() {
  int N, M;
  cin >> N >> M;
  vector<int> s(M);
  vector<int> c(M);
  vector<vector<bool>> A(N, vector<bool>(10, false));
  for (int i = 0; i < M; i++) {
    cin >> s[i] >> c[i];
    A[s[i] - 1][c[i]] = true;
  }
  for (int i = 0; i < N; i++) {
    if (i == 0) {
      if (A[0][0] == true) {
        cout << -1 << endl;
        return 0;
      }
      int B = 0;
      for (int j = 1; j < 10; j++)
        if (A[i][j] == true) B++;
      if (B > 1) {
        cout << -1 << endl;
        return 0;
      }
    } else {
      int B = 0;
      for (int j = 0; j < 10; j++)
        if (A[i][j] == true) B++;
      if (B > 1) {
        cout << -1 << endl;
        return 0;
      }
    }
  }
  int answer = 0;
  for (int i = 0; i < N; i++)
    for (int j = 0; j < 10; j++) {
      int d = pow(10, N - i - 1);
      if (A[i][j] == true) answer += j * d;
    }
  cout << (answer < 100 ? answer + 100 : answer) << endl;
}
