Adding up profit and loss

I'm running the accounts for a village fair, and I'm stuck. I have a list of payments into and out of an account. The list looks a bit like this:
A...................B
Sum....| Profit or loss (P/L)
10........| P
12........| L

I then want to add up all the 'P' items and all the 'L' items separately. I tried VLOOKUP, but it doesn't work.

How do I do it?