Excel 2010 Calculated Fields

[Graeme]

Hello,

I am trying to summarise some accounting data against last year using a
pivot table. My pivot table currently looks like this:
Jul09 Aug09 Sep09 Jul10 Aug10 Sep10
Division a b c e f g

I would like to add the following calculated fields:
e-a
f-b
g-c
sum(e,f,g) - sum(a,b,c)
Is there a way to do this? The individual months don't appear within the
calculated fields dialog box.

My database has the following format:

Division Period Amt
Coy1 Jul09 a
etc

Thanks,

Graeme.