Using slicer values in measures

Hi, 

I need to calculate the average of a performance indicator depending on a slicer value that represents a period. Let's say the user chooses period 100, I need to calculate the average of the performance indicator for periods 96,97 and 98. So I tried to extract the value of the slicer with Selected Value and then substract 2 to that measure to get the 98 I wanted, but when I add that measure into my average measure it shows blank. Although, when I write 98 manually into the formula, it works.

What have I done wrong? 

Thanks in advance.