#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<char> str;
  for (int i = 0; i < n; i++) {
    char x;
    cin >> x;
    str.push_back(x);
  }
  int i;
  for (i = 0; i < n - 1; i++) {
    if (str[i + 1] != str[i]) {
      cout << "YES" << endl;
      cout << str[i] << str[i + 1] << endl;
      return 0;
    }
  }
  if (i == n - 1) cout << "NO" << endl;
  return 0;
}
