#include <bits/stdc++.h>
using namespace std;
int j = 1, f = 0;
int main() {
  int n, m, l, arr[1000];
  cin >> n;
  string str, str1;
  cin >> str;
  for (int i = 0; i + 1 < n; i++) {
    if (str[i] != str[i + 1]) {
      cout << "YES" << endl << str[i] << str[i + 1];
      j = 5;
      break;
    } else
      f = 1;
  }
  if (j != 5) cout << "NO";
}
