Calculated Item or Field Help

I've attached a sample file. On it I have a pivot table showing our vendors 2013 and 2014 sales and I can filter it by Sales Rep. I need the pivot table to also give me the "Dollar +/-" between the two years. I can't get the calculated item or calculated field to work. Can this be done on the pivot table itself? I know I can create a calculation in column D on the "pivot" tab with the formula but I'm trying to automate the task.

I need to take that data and create one tab per sales rep and list the top 15 "dollar +/-", the bottom 15 "dollar +/-", and want to get this all to update easily when I enter a new months worth of data.

Thanks for your help.

I wasn't able to do that in my actual file but did it instantly in the my sample file I uploaded here. After some research I was able to make a few changes to my actual file to get it to work. Thanks for your help.