#include <bits/stdc++.h>
using namespace std;
vector<int> forintinput(int n) {
  vector<int> x(100000);
  for (int i = 0; i < n; i++) cin >> x[i];
  return x;
}
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[i] >> c[i];
  }
  for (int i = pow(10, (n - 1)); i < pow(10, n); i++) {
    string u = to_string(i);
    int cnt = 0;
    for (int i = 0; i < m; i++) {
      if (u[s[i] - 1] != c[i]) {
        cnt++;
      }
    }
    if (i == pow(10, n) - 1) {
      cout << -1 << endl;
      break;
    }
    if (cnt != 0) {
      continue;
    } else {
      cout << i << endl;
      break;
    }
  }
}
