#include <bits/stdc++.h>
using namespace std;
int main() {
  ;
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long int n, i;
  string str;
  bool result = false;
  cin >> n;
  cin >> str;
  for (i = 0; i + 1 < str.size(); i++) {
    if (str[i] != str[i + 1]) {
      result = true;
      break;
    }
  }
  if (result) {
    cout << "YES\n";
    cout << str[i] << str[i + 1] << "\n";
  } else
    cout << "NO\n";
}
