#include <bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < (n); ++i)
using namespace std;
using ll = long long;
using P = pair<int,int>;

int main() {
  int N, M;
  cin >> N >> M;
  vector<int> value(3, 0);
  vector<bool> change(3, false);
  rep(i, M){
    int s;
    int c;
    cin >> s >> c;
    if(change.at(N-s) == false){
      value.at(N-s) = c;
      change.at(N-s) = true;
    }
    else if(value.at(N-s) == c){
      value.at(N-s) = c;
    }
    else{
      cout << -1 << endl;
      return 0;
    }
  }
  int ans = value.at(0) + value.at(1)*10 + value.at(2)*100;
  int tmp = ans;
  int digit = 0;
  if(tmp < 10){
    digit = 1
  }
  else if (tmp < 100){
    digit = 2;
  }
  else{
    digit = 3;
  }
  if(digit!=N) cout << -1 << endl;
  else cout << ans << endl;
  return 0;
}