#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  string s;
  cin >> s;
  char c = s[0];
  string str = "";
  str += c;
  bool ans = false;
  for (int i = 1; i < n; i++) {
    if (s[i] == c) {
      continue;
    }
    ans = true;
    str += s[i];
    break;
  }
  if (ans == false)
    cout << "NO" << endl;
  else
    cout << "YES" << endl << str << endl;
}
