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