A cafe loyalty card makes every fifth purchase free: the 5th, 10th, 15th, ... coffee costs
nothing, whatever its price. prices lists the prices (in cents) of a customer's coffees in the
order they were bought. Return the total amount the customer actually paid.
Examples
Input: prices = [300, 300, 300, 300, 300, 300]
Output: 1500
Input: prices = [400, 500, 600, 700, 10000]
Output: 2200
Constraints
0 <= len(prices) <= 1000,0 <= prices[i] <= 10**5
Goals
- Use a 1-based position inside a loop to spot every fifth item
- Sum only the items that must be paid for