whats is an excel calculation for: for every two items they get $5 off?

How do I create an excel function if I want to calculate for every second item they purchase they get $5 off
So I input the number of quantity a customer purchased, and it will calculator for me the price they have to pay. Each item is $20. The discount rate is for every two items, they get $5 off. (1=$20, 2=$35, 3=$55, 4=$70)

Thanks!