#include <bits/stdc++.h>
using namespace std;
const int INF = 100100100;
const int MOD = 1e9 + 7;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  ;
  int N, M;
  cin >> N >> M;
  vector<int> s(N + 1);
  vector<int> c(N + 1);
  map<int, int> check;
  for (int i = 0; i < (M); i++) {
    cin >> s[i] >> c[i];
    decltype(check)::iterator it = check.find(s[i]);
    if (it != check.end()) {
      if (it->second != c[i]) {
        cout << -1 << endl;
        return 0;
      }
    } else {
      check.emplace(s[i], c[i]);
    }
  }
  decltype(check)::iterator it = check.find(1);
  if (it == check.end() || it->second == 0) {
    cout << -1 << endl;
    return 0;
  }
  string ans = "";
  for (int i = (1); i < (N + 1); i++) {
    decltype(check)::iterator it = check.find(i);
    if (it != check.end()) {
      ans += to_string(it->second);
    } else {
      ans += "0";
    }
  }
  cout << ans << endl;
}
