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