#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m;
  cin >> n >> m;
  pair<int, int> k[m];
  int x, y;
  if (m == 0) {
    if (n == 1) {
      cout << 0 << endl;
    } else {
      cout << -1 << endl;
    }
  } else {
    for (int i = 0; i < m; i++) {
      cin >> x >> y;
      k[i] = make_pair(x, y);
    }
    string s = "";
    for (int i = 0; i < n; i++) {
      s += 'a';
    }
    bool ok = true;
    sort(k, k + m);
    for (int i = 0; i < m - 1; i++) {
      if (k[i].first == k[i + 1].first && k[i].second != k[i + 1].second) {
        ok = false;
        break;
      }
      s[k[i].first - 1] = k[i].second + '0';
    }
    s[k[m - 1].first - 1] = k[m - 1].second + '0';
    if (s[0] == '0') {
      ok = false;
    } else if (s[0] == 'a') {
      s[0] = '1';
    }
    if (!ok) {
      cout << -1 << endl;
    } else {
      for (int i = 1; i < s.length(); i++) {
        if (s[i] == 'a') {
          s[i] = '0';
        }
      }
      cout << s;
    }
  }
}
