How can I run a runnin delta total on a pivot table instead of a sum?

the operation instead of sum would be from column Type D-S
and the totals shold be the result of the first week plus next etc.

Sum of Qty WK WK of
PastDue 1004 1005 1006 1007
Part # Type pastdue 1/23/2010 1/30/2010 2/6/2010 2/13/2010
part1 D 14010
S 1000 1000
part1 Total 14010 1000 1000
part2 D 360
S 180
part2Total 360 180
part3 D 600
S 600
part3 Total 600 600
part4 D 7980 2580
S 2040
part4 Total 7980 4620