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