SQL Server - calculated column

I need to create a column in a view which is derived as below.

The calculated column is derived from Amount column where a row value is derived by subtracting from previous row value of Amount, and every 1st contract that starts will have no value as it's the first row of that contract.

Can someone please suggest what option I have to achieve this?

Any suggestions are welcome.

This is a way to do it using window function LAG() to get the previous value :

select *, Amount - lag(Amount) over (partition by Contract order by DateValue) as [Calculated Column]
from mytable

Demo here

Your data has no way to determine the order of duplicate DateValues so I will assume you also have an identity id column.
(A relation is an unordered set!)

Why does 200 - 250 = 50?

SELECT id, [Contract], DateValue, Amount
   ,Amount -
      LAG(Amount) OVER (PARTITION BY [Contract] ORDER BY DateValue, id) AS CalcValue
FROM YourTable;