Difference between two file totals

Hi everybody,

I have a question that could solve me many different questions about comparing two files/weeks.

Practically I have two file -> SalesPresentWeek and SalesPreviousWeek

Now. I need to represent in a matrix or table the delta between Present and Previous week.

I need to build a table like this (counting elements in Present week)

After this I need to calculate the same for the Previous week and put the delta in this table without visualize the previous week values.

Let assume tha UT Trapani have now 184 counts. Previuous week had 194 count. I want to see -10 in this column.
Actually I build a measure 

but gives me wrong numbers

How I can do that correctly?

Thank you