Cumulative formula - I cant get something right - see file -

Please see the attached file....

What I am trying to do is:

If I buy and sell a quantity at a certain price; it spits out the profit/loss
this is easy when you buy sell, but when you buy,buy,buy, then sell all or a portion of it; it gets complicated.

If I buy 100 items, then buy another 100, then sell 200. how do I make sure it knows the price of the first 100? PLEASE HELP

See attached - The yellow is what you enter, the blue and red is formulas.