#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  string s;
  cin >> s;
  string ans = "";
  map<char, int> mp;
  for (int i = 0; i < n - 1; i++) {
    if (s[i] != s[i + 1]) {
      ans += s[i];
      ans += s[i + 1];
      break;
    }
  }
  if (ans.size() == 0) {
    cout << "NO";
  } else {
    cout << "YES" << endl;
    cout << ans;
  }
  return 0;
}
