Repeat a formula after every nth column

Hello All,

I am trying to create a formula by which I can calculate sales of a product after every n months.

For example

Inputs:
Number of Units = 10
Price per unit = 50
Frequency of sale = every 6 months
First sale in 2nd month.

Output:
Month 2 = 500
Month 8 = 500

Example is attached for refrence.

Thank you for the help.