Can Tablix display DIFF rather than SUM in "Total" column

Hi All,

Is it possible to display the difference between 2 cols rather than SUM when we addTotal column to a Tablix ?

In the attached tablix where it says Total I want to display difference in values over 2 years rather than just summing them up.

Or is there any other alternative - like any built-in SSRS expressions ?

http://imageshack.us/photo/my-images/195/dataic.png/