#include <bits/stdc++.h>
using namespace std;
const int mod = 998244353;
int main() {
  long long n;
  cin >> n;
  string s;
  cin >> s;
  for (int i = 0; i < n - 1; ++i) {
    if (s[i] != s[i + 1]) {
      cout << "YES"
           << "\n";
      cout << s[i] << s[i + 1];
      return 0;
    }
  }
  cout << "NO";
  return 0;
}
