#include <bits/stdc++.h>
using namespace std;
void solve() {
  int n;
  string s;
  cin >> n >> s;
  for (int i = 0; i < n - 1; i++) {
    if (s[i] != s[i + 1]) {
      cout << "YES"
           << "\n";
      cout << s[i] << s[i + 1] << "\n";
      return;
    }
  }
  cout << "NO"
       << "\n";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  solve();
  return 0;
}
