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