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