#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m;
  vector<int> a, b;
  int ans = 0;
  cin >> n >> m;
  if (n > 1) {
    vector<int> nv(n, -1);
    bool ansF = true;
    for (int i = 0; i < m; i++) {
      int k, v;
      cin >> k >> v;
      k = k - 1;
      if (nv[k] == v || nv[k] == -1) {
        nv[k] = v;
      } else {
        ans = -999;
      }
    }
    if (ans != -999) {
      for (int i = 0; i < n; i++) {
        if (i == 0 && nv[i] == 0) {
          cout << -1 << endl;
          ansF = false;
        }
        if (nv[i] == -1) {
          if (i == 0) {
            nv[i] = 1;
          } else {
            nv[i] = 0;
          }
        }
        ans = (ans + nv[i]) * 10;
      }
      ans = ans / 10;
      if (ans) {
        cout << ans << endl;
      }
    } else {
      cout << -1 << endl;
    }
  } else if (m > 0) {
    int kx, vx;
    cin >> kx >> vx;
    cout << vx << endl;
  } else {
    cout << 0 << endl;
  }
}
