Problem Statement  
You want to buy a certain set of items. The shop offers you a discount percent discount on every third item you buy. Therefore, you want to purchase the items in an order that will minimize the total cost. Given a int[] prices containing the prices of the desired items and an int discount, return the minimal amount of money you must have in order to buy all the items.  
Definition  
 
Notes  
  Your return value must have an absolute or relative error less than 1e9.  
Constraints  
  discount will be between 0 and 100, inclusive.  
  prices will contain between 0 and 50 elements, inclusive.  
  Each element of prices will be between 0 and 2147483647, inclusive.  
Examples  
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