Adding a Variance row to a pivot table?

Having trouble figuring this out. It may not even be possible. I am trying to add a variance row to a pivot table. The table shows revenue for 2012 and 2013 by month. I want to add a column under 2013 that shows the difference (2013-2012).

I know you can add custom calcs between different columns but not sure if you can for values within a column. The attached images show what i am trying to do. Is this possible? :eek:

Attached Images

	excel1.PNG (6.9 KB)
	excel2.PNG (7.1 KB)