#include <bits/stdc++.h>
using namespace std;
const int oo = 1e9 + 7;
const int N = 1e6 + 10;
string s;
int n;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cin >> n;
  cin >> s;
  for (int i = 1; i < s.size(); i++) {
    if (s[i] != s[i - 1]) {
      cout << "YES\n" << s[i - 1] << s[i];
      return 0;
    }
  }
  cout << "NO";
}
