Difference between previous week and this week data

[ApurvaKhatri]

Hello,

I have data as follows

Date      Sales

Oct 4 -  36

Oct 11 - 30

Oct 18 - 25

Oct 25 - 20

I need the difference between this week and previous week data i.e 

Oct 25 - Oct 18

20 - 25 = (5)

So I need 

Difference -

(5)

(5)

(6)

.... to be displayed

How can I acheive this?

Thank you for your help

Apurva Khatri