How to subtract consecutive columns in a crosstab report in iReport

I have developed a crosstab report in iReport. I want to subtract values from consecutive column values. How can I do that??

For e.g. If the cross tab report shows the following:

       Jan    Feb
Salary 10000  20000
Basic  5000   10000

But I want to show it like:

       Jan    Feb
Salary 10000  10000 (Feb - Jan data)
Basic  5000   5000  (Fe开发者_开发知识库b - Jan data)