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