i have created an excel sheet and assign a pivot table to it , in the pivot table it shows total sum of invoicing & retailing. below is the eg.

This is what my Pivot dashboard shows

Product Name| invoicing | retailing |Total Sum
Product 1 | 12000 | 10000 |22000
Product 2 | 15000 | 16000 |31000
Product 3 | 10000 | 10000 |20000

What i actually want is the (Invoicing – Retailing ) value in total sum.eg (12000-10000 = 2000)

Please Help.

I tried doing it manually on the dashboard itself , i know i am doing it in a wrong way. enter image description here

this is what i did to all the rows manually , but when i change the month & it gives error for other rows.