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