#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, a;
  char arr[1009];
  cin >> n;
  cin >> arr;
  for (a = 0; a < n - 1; a++) {
    if (arr[a] != arr[a + 1]) {
      cout << "YES" << endl;
      cout << arr[a] << arr[a + 1];
      return 0;
    }
  }
  cout << "NO";
}
