Doing a calculation in a pivot table

Hi all,

This one has been bugging me for a long time so hope you can help.

I have a pivot table which shows two columns of data; Week 1 and Week 2.

At the moment the "Grand Total" column adds the two columns up. Is it possible to show the variance in this "Grand Total" column, i.e Week 1 = 10, Week 2 = 4, Variance = 6.

Any advice on possible solutions would be great.

Thanks
Garry