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