Pivot Tables- finding difference between two values of same fields

[rdev]

I put up a database which summarises sales on a weekly basis . In column A are listed the products , in column B , I have the sales quantities , in column C i have the week . I have two weeks data , that of, say week 9 and week 10. I put up a pivot table, with the products in the row quadrant , the weeks in the column quadrant, the sales in the data quadrant . Now I want to get in the 4th column of my pivot table ,a field with the difference between week 9 and week 10. I have attempted calculated field , it does not work as it is the same field that i minipulate. can someone help me .
Thks