#include <bits/stdc++.h>
using namespace std;
long long arr[23];
int main() {
  int n, div;
  cin >> n;
  cin.ignore();
  string s;
  cin >> s;
  for (int i = 1; i < n; i++) {
    if (s[i] != s[i - 1]) {
      cout << "YES" << endl;
      cout << s[i - 1] << s[i];
      return 0;
    }
  }
  cout << "NO";
  return 0;
}
