Trying to calculate the following:

For each item sold for the month a credit is awarded. I'm having a difficult time to calculating the credits due to the way the credits are awarded.

The credit for selling an item is not a 1 for 1. It is compounded as follows:

-If 1 item is sold for the month = 2 credits are awarded (easy).
-If 2 items are sold for the month = 4 credits are awarded in addition to the 2 credits awarded for the first item sold. (2+4 =6 credits)
-If greater than 2 items are sold for the month = 6 credits are awarded for EACH item sold above 2. So the third item sold would be an additional 6 credits added to the previous sales credits (2+4+6 = 12 credits). 6 items sold would be (2+4+6+6+6+6=30 credits)

So I'm trying to calculate total credits awarded at the end of the month.
Using 2 cells (lets say C1=sales and C2= credits) I'm having trouble with the formula.

I would also like to break it down by month. Each column representing a month, whereby a total credit would show at the end of a row (a row representing a name.
Example Row C =(name), JAN (C1=sales, C2+Credits) + FEB (C3=sale, C4=Credits) + etc etc. = Total credits (Cxx)
I have not made a sheet yet with all the months. Just trying to figure out the 2 cells (sales and credits)

I have tried using IF AND. Not having any luck with example formulas on the web.

Any help would be appreciated.

I did post in another forum