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Hunt for discount

Story

My local pizza delivery introduced new discount. You get 50% discount from every second item on your order.

But being greedy capitalists, they forgot to mention that they will rearrange items the way they need to give you as little as possible.

Example

Imagine you ordered

- Pizza $20
- Pizza $20
- Coke $2
- Coke $2

You expect to get $10 discount from the second pizza and $1 from the coke, but they rearrange it as

- Pizza $20
- Coke $2
- Pizza $20
- Coke $2

and give you $2.

Trick

Later I noticed that I can place as many orders as I want simultaneously, so I just split my order into two:

 1. 
 - Pizza $20
 - Pizza $20
 2. 
 - Coke $2
 - Coke $2

and I got the discount I deserve.

Problem

Can you please help me to write a program that calculate the maximum discount I can get by splitting my order.

It should accept a list of prices and return an amount.

For simplicity, all prices are even numbers, so the result is always an integer. Order is never empty. Input have no specific order.

This is code golf, do all usual rules applies.

Testcases

[10]             -> 0
[10,20]          -> 5
[10,20,30]       -> 10
[2,2,2,2]        -> 2
[4,10,6,8,2,40]  -> 9

Answer*

# Java, 86 bytes



    L->{int s=0,i=L.size();for(L.sort((a,b)->a<b?1:-1);i-->0;)s+=i%2*L.get(i);return s/2;}

[Try it online.](https://tio.run/##ZZBRa8IwFIXf/RUXYZDMNNNOxrC2Y@xp0Pni49hDWqPE1USSWzcn/vYubdXpJBC4J19yTs5SbESwnH1WeSGcgzeh9K4DoDRKOxe5hEk9NgLkZOlpXqIqeKocjl89tZA2gZRGntp3/OZQoMphAhriKg2SXX3TxX2m4pQ79SMJjebGEj8Yi4QIltEgEePsaTAKBjRSQZD0I@p6sboJb1O@kEgUjazE0mpwd2G0r6LaaF1mhTc6@G2MmsHKpydTtEov3j8EbZOjdEgG/SbhaWLhtcDuL7SQNetcGjIPPrBHrw8v2eb@kT7vocnVML4Hzvkx1UZY3@m6xLpIiEHLL/hr99lasb2omPw7dNyhlWJFBOWZ@ZYzQnluikLmeIa2DH9pD4x1HE39LKH0EH@6dShX3JTI1743LPS1E5q2U@/V646g29M8J6fw9PDnffUL)

**Explanation:**

    L->{                      // Method with Integer-List parameter and Integer return-type
      int s=0,                //  Sum-integer, starting at 0
          i=L.size();         //  Index-integer, starting at the size of the input-List
      for(L.sort(             //  Sort the input-List
            (a,b)->a<b?1:-1); //  in reversed order
          i-->0;)             //  Loop `i` in the range (size,0]:
        s+=                   //   Increase the sum by:
           i%2                //    Check if the current `i` is odd (1 if odd; 0 if even)
           *L.get(i);         //    And multiply that to the current `i`'th value
      return s/2;}            //  After the loop, return halve the sum

If this is an answer to a challenge…

…Be sure to follow the challenge specification. However, please refrain from exploiting obvious loopholes. Answers abusing any of the standard loopholes are considered invalid. If you think a specification is unclear or underspecified, comment on the question instead.
…Try to optimize your score. For instance, answers to code-golf challenges should attempt to be as short as possible. You can always include a readable version of the code in addition to the competitive one. Explanations of your answer make it more interesting to read and are very much encouraged.
…Include a short header which indicates the language(s) of your code and its score, as defined by the challenge.

More generally…

…Please make sure to answer the question and provide sufficient detail.
…Avoid asking for help, clarification or responding to other answers (use comments instead).

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