Sum up key figures in cube

Hi,

I would like to include an ABAP code in a start routine.

My infocube contains account numbers. for each account there is an amount (0QUANTITY) and a 0CALMONTH. Each line in the cube contains an account number and a 0CALMONTH.

On every line of my infocube (for every account number with its 0CALMONTH) I would like to calculate in a new column the total amount for a duration of a roll back year. Ex: Account Number 123, 0CALMONTH = 01/2009, 0QUANTITY = 10 -> In the new colomn I want the sum of 0QUANTITY for a roll back year (0QUANTITY 01/2009 + 0QUANTITY 12/2008 + 0QUANTITY 11/2008 + ... + 0QUANTITY 02/2008).

Can someone give me an example of the ABAP code to perform this?

Thanks a lot!