Difference between colums in pivot

Hello, I have an excel sheet with a pivot table with some data about the trades that I have done in the stock market. Before reading the query request you to please have a look at the attachment as I have referred a few data columns.

Coming to the pivot table, in column 'I', I have the 'Total Sum of Trade Value' column which basically adds up the data frome columns E and G (i.e. Sum of trade values under 'Buy' and 'Sell'). I also have the brokerages paid for buying as well as selling and the brokerages are summed up in column J. Basically, what I want to do is to be able to get a difference between different columns within a pivot table. So I want a column which will have 'G-E-J' (Sum of Sell value - Sum of Buy value - Sum of Brokerage). I could click on one of the data fields and after selecting Value filed settings, it shows the summarized by tab. However here, I could only see functions like, Sum, Count ..etc ..but Difference isn't there PLease help !!!!