Dax measure

Hi 

I am using matrix table and there is sales1 and sales2, iam dynamically selecting those measures from drop down slicer

I have used "difference "= sales1-sales2 which runs when both are selected from drop down colums..

I want show difference to" blank"when sales2 not selected  in the table from the drop using measure

Used difference = if(isblank(sales2),blank(),sales1- sales2

But not getting blank value when there is no sales2 measure in the table