#include <bits/stdc++.h>
using namespace std;
int n;
string s;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cin >> n;
  cin >> s;
  if (n == 1) {
    cout << "NO";
    return 0;
  }
  for (int i = 0; i <= n - 2; i++) {
    if (s[i] != s[i + 1]) {
      cout << "YES" << endl << s[i] << s[i + 1];
      return 0;
    }
  }
  cout << "NO";
  return 0;
}
