Subtraction of previous period in PowerQuery

I have 2 tables in power query which I would like to append. But before appending these 2 tables I would like to implement the following subtraction in power query. How to get around with this ? many thanks in advance.

C.Code  Amount  Date    FiscalYearPeriod    Expected Column 
1001    5   15/01/2024  202401              0-5   = -5
1001    10  15/02/2024  202402              10-5  =  5
1001    20  15/03/2024  202403              20-10 = 10
1001    40  15/04/2024  202404              40-20 = 20
1001    80  15/05/2024  202405              80-40 = 40
1002    100 15/05/2024  202405              0-100 = -100

you can try this

= Table.AddColumn(#"Changed Type", "Custom", each 
[a=Table.SelectRows(#"Changed Type",(x)=>x[C.Code]=[C.Code]and x[Date]<[Date]),
b= try [Amount]-a[Amount]{Table.RowCount(a)-1} otherwise 0-[Amount]
][b])