Value difference by index in rows

Hello,

I am facing this issue that I need to get difference between rows by index number.

Data i have -->

So row with index 31 should return 4 (338-334), row with 32  index should return 3 (341-338), row with 33 index should return 1 (342-341) and so on and so on. In my data Index is grouped by ak_id. It seems a simple task, but everything i tried i get blank values. 

Maybe anyone has any ideas on how to do this. It doesn't matter it's measure or cc.

Thank you!