Billing algorithm

This problem is from Codechef. I am re-wording it for more clarity.

A garments shop has “Buy 2, Get 2” offer for it’s customers. Whatever the items may be, they charge for costliest items from an order and give the cheapest ones free.
However we can divide the items into multiple orders to save more.

For example, Let us say we have 6 items with prices 200, 600, 100, 900, 800, 700. If we take them as one order, we will have to pay 3000.

6 items are divided into two groups {900,800,200,100}, {700,600} because they charge for highest price items. So the total bill would be 900+800+700+600 = 3000
Suppose we divide the items into two groups {900,800,700,600}, {200,100} we need to pay only 900+800+200+100 = 2000

Now the problem is, given a list of prices, how do we calculate the minimum amount we need to pay?

This is a simple greedy algorithm. We just need to sort all the prices in descending order and group them into 4 per order. The last order might contain less than 4 items.

Here is the C++ code for the same