Here's the table for one game

Date-----Buy(Unit)-----Sell(Unit)-----Net Unit-----Price($)-----Net
Profit/Loss($)
xx/xx----2----------------##--------------2--------------125---------- ##
xx/xx----##---------------1---------------1--------------150---------- ##
xx/xx----##---------------1---------------0--------------165---------- +65
xx/xx----##---------------4--------------(4)-------------170--------- ##
xx/xx----2----------------##-------------(2)-------------165---------- ##
xx/xx----##---------------1--------------(3)-------------180---------- ##
xx/xx----3----------------##--------------0--------------180---------- (10)

Number in bracket means negative.
## means empty cell.

How can I type a formula, so that every time there's no stock left (ie zero net
unit), it will calculate the net profit once.
Any workaround is also appreciated.
Thank you.