Excel Formula Spreadsheet

Hello,
I'm trying to create a workbook that will keep an account of inventory buys and sells I do on a daily basis.
The spreadsheet already includes:
Buy or Sell/Number of Products/Price
now I need a separate cell that shows net position and average price.

So if I sell 15 units and then later buy 5 units, I will be short 10 units. I need to account for this as well as average price from the data below.

B/S # of products Price
S -15 100.00
B 5 50.00

I still have an open short position of -10 in inventory with an average price of 100.00. I also have a gain of 50.00 from the 5 I bought for inventory. This formula will need to include both short sales as well as long sales with an average price.

Can I please get some help on this?