#include <bits/stdc++.h>
using namespace std;
bool alsmae(string s) {
  long long n = s.length(), i, flag = 0;
  for (i = 0; i < n - 1; i++) {
    if (s[i] != s[i + 1]) {
      flag = 1;
      break;
    }
  }
  if (flag == 1) {
    return false;
  } else {
    return true;
  }
}
signed main() {
  long long t = 1;
  while (t--) {
    long long i, n;
    string s;
    cin >> n;
    cin >> s;
    if (alsmae(s)) {
      cout << "NO" << '\n';
    } else {
      for (i = 0; i < n - 1; i++) {
        if (s[i] != s[i + 1]) {
          cout << "YES\n";
          cout << s[i] << s[i + 1] << '\n';
          break;
        }
      }
    }
  }
}
