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