Average based on date slicer

Hi all,

I have created a table like this

Sales table

I created a date slicer 

And I want to create a card that would show the average based on the selected period

So,

When I select Jan, Feb, and March in the slicer

Sales Jan = 100 + 200 + 500 = 800

Sales Feb = 150 + 200 + 400 = 750

Sales March = 300 + 160 + 450 = 910

I want the card showing the average sales for 3 months = (800+750+910)/3 = 820

When I select Jan, Feb, and March in the slicer

Sales Jan = 100 + 200 + 500 = 800

Sales Feb = 150 + 200 + 400 = 750

Sales March = 300 + 160 + 450 = 910

Sales April= 220+ 210 + 500 = 930

I want the card showing the average sales for 4 months = (800+750+910+930)/4 = 847,5

Please help me to solve this problem

Thank you..