#include <bits/stdc++.h>
using namespace std;
int main() {
  char s[1010];
  int n;
  cin >> n;
  cin >> s;
  for (int i = 0; i < n - 1; i++) {
    if (s[i] != s[i + 1]) {
      cout << "YES" << endl;
      cout << s[i] << s[i + 1];
      return 0;
    }
  }
  cout << "NO";
  return 0;
}
