Calculate difference within one column

[Koda7]

I have a basic list query set up (in black):

Week #       Amount         WTW Chg
1                  234                  0
2                  343               109
3                  422                79
4                  489                67
5                  589               100

As the weeks go forward, the amount will always grow.  I want to put another column in to calculate the week to week change that should look like the 3rd column in Red. 

[blom0344]

I think that you may find the 'running-difference' function appropriate..