#include <bits/stdc++.h>
using namespace std;
int main() {
  long n;
  cin >> n;
  char ar[n];
  cin >> ar;
  bool f = false;
  for (long i = 0; i < n - 1; i++) {
    if (ar[i] != ar[i + 1]) {
      cout << "YES" << endl << ar[i] << ar[i + 1];
      f = true;
      break;
    }
  }
  if (f == false) cout << "NO";
}
