#include <bits/stdc++.h>
using namespace std;
long long n, i, j, fl;
string s, s1;
int main() {
  cin >> n >> s;
  for (i = 0; i < s.size() - 1; i++) {
    s1 = s.substr(i, 2);
    if (s1[0] != s1[1]) {
      cout << "YES"
           << "\n"
           << s1 << "\n";
      return 0;
    }
  }
  cout << "NO"
       << "\n";
  return 0;
}
