How To Generate An Expression That Uses The Value From A Previous Column In A Matrix???

I have a RS Matrix element where I am attempting to genrate an expression that uses a value from a previous column. I have had no luck so far...

Here's a brief example of the matrix that I am using:

=Fields!MonthName (my one and only Column Group)

=Fields!ProductName =Sum(Fields!Count) =DifferenceExpressionInQuestion

I gotta use a Matrix since the number of months is dynamic.

In my test, I have 12 month columns. Under each Month column is a subordinate Count and a Difference column. In the Difference column, I want to calculate the difference in the Count columns relative to the previous month (if the Count for Jan is 15 and the Count for Feb is 27, then Feb's Difference column should be 12).

So, I am stuck trying to generate an expression for the Difference column. This expression needs to refer to the Count of the previous month.

RS's Previous() function cannot be used in a Matrix (because that's the compiler error that I get when I try to use it).

Does anybody know how to do this???

Do I have to use custom code to do this???

If I can't do this calculation in a matrix, then what good is it???

TIA!!!