Counting previous row and totals

[#1]

Hi i have a situation whereby i would like to accumulate in my column and present the total in the totals row.

I have 3 expressions

1. Order

2. Stock

3. Purchase Orders

4. Available Stock is calculated as follows (Stock - Order + Purchase Orders)

Stock is always at the level that you see now so for example 50 for Art No. 100.

Art No.	Date	Stock (right now)	Order	Purchase Order	Available Stock
100	2013-10-20	50	200	0	-150
100	2012-11-20	50	300	200	-250
Total Art No	-	50	500	200	-250
200	2013-09-15	30	25	10	15
200	2013-10-20	30	10	0	5
Total Art No	-	30	35	10	5

Best Brad