#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  string s;
  cin >> n;
  cin >> s;
  int counter = 0;
  string diverse;
  for (int i = 0; i < n; i++) {
    if ((i + 1) < n && s[i] != s[i + 1]) {
      diverse += s[i];
      diverse += s[i + 1];
    }
    if (diverse.length() >= 2) {
      break;
    }
  }
  if (diverse.length() >= 2) {
    cout << "YES" << endl;
    cout << diverse;
  } else {
    cout << "NO";
  }
}
