HELP!! Calculation based on drop down list??

Hi Guys!

I havent been using excel for too long, needed some help?

The page is split into two sides:

Goods In/ Goods Out

When i select a product from the goods in side and add a quantity,I would like to add to the main list on the far left list?

and when i add a product on the goods out side and add a quantitiy, I would like it to be subtracted from the list on the far left?

Ive been going crazy to find a suitable formula for this but somehow I cant find one?

ANY IDEAS?