Pivot: Create a variance column

How can I create a calculated column that is the variance between
column A and column B data? I can do this in Excel's pivot by
inserting a calculated item or field (can't remember which one). I've
tried the create "calculated total" and "calculated detail field"
options in the Pivot table view, but always get an error.

This is my pivot table:
Rows: Account (shows all accounts)
Col: Category (shows two categories)
Data: Amount

I'd like the columns to show:
Cat1 | Cat2 | Variance

How can I insert the "Variance" column?

Thanks for any suggestions!