Front-End issue

Hello, I have a query about a report design. This is what my report looks like:

Material Fiscal Period Begin No. RKF1 RKF2 Difference

101 01/2014 100

101 02/2014 200

101 03/2014 300

101 04/2014 500

Currently I am doing RKF2 - RKF1 and printing result in difference column

Material Fiscal Period Begin No. RKF1 RKF2 Difference

101 01/2014 100

101 02/2014 200 -200

101 03/2014 300 300

101 04/2014 500 -500

My requirement is to make my difference as cumulative ok so it should be like this:

Material Fiscal Period Begin No. RKF1 RKF2 Difference

101 01/2014 100 100

101 02/2014 200 300 because (100 + 200)

101 03/2014 300 then I should take 300 + 300 = 600

101 04/2014 500 then I should take 600+500 = 1100

so the ending number in my difference should be 1,100. This is a custom keyfigure I am using. I think this concept is in Inventory where you take beginning balance etc. but how can i implement that concept in my custom keyfigure?

Please suggest a way.

Thanks.