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