Calculate 12 month performance from date

[rki1966]

How to I calculate the 12 month performance based on the next month end as start date

Example: 03/31/10 is the key

column A is the month end and column b is the month return.

Cell A20 = 03/31/10
Cell A21 = 04/30/10


So the formula would be like {=PRODUCT(1+B21:B32)-1}

I would also like to chnage the period based on a cell: Currently is set to 12, if I chnage to 5 then the formula would be {=PRODUCT(1+B21:B25)-1}