subtracting recent data from previous data

Hi,

I need some help with an app that i am working on. i have a csv file that one of our user is uploading and populating data in the database every month. 

here is an example of what it looks like a clear picture is attached.

Revenue	Amount	Month	Year
USD Int on FT Deposit	2035	january	2016
USD Int on Treasury Bills	34523432	january	2016
AUD Int on Bond	43243	january	2016
Total Int on Foreign Asset	4324	january	2016
Dealing Profit	4324	january	2016

what i want to accomplish is, every time when the user upload and populate database. I want to get the recent amount that was just upload  and minus the previous amount.

for example: when the user upload and populate data from February, i want to get amounts of February and minus the January amounts. i want do that for all the rows.

I've already build the done the uploading part, just need some help on the calculation part.

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