Inventory formula

I am trying to create an inventory formula for an excel spread sheet.
inventory
ZXT ZXT GOLD ZXT SLIM
IN HOUSE
DATE IN OUT IN OUT IN OUT
11/9/2012 300
11/10/2012 58

in the tow for in house is where i want the running count to be. basically everyday when a product is removed the date will be inserted into the columns A/B and the amount will be inserted in column (c/d or e/f, or g/h) under in or out.

i cannot figure out what formula to use…
any ideas?