Issue: Difference column repeats after every month.

I have a matrix in my report where

• Column group = month
• Row = service
• Values = cost

I've inserted a column (inside group - right) in my column group "month". I have the expression working for the difference between the current month cost - previous cost using the below expression.

sum(Fields!Cost.Value) - Previous(sum(Fields!Cost.Value), "month_group")

The problem is that this difference column repeats after every after month column like:

January | Difference | February | Difference | Total

How to avoid this setup?