How to include +ve and -ve amounts

Hello,

I am working on a report to display the Additions and Deletions of an asset based on document ids(a,b,c etc).

Additions column should populate the amount from document id=A+ only positive amounts  from document ids B and C, similarly Deletions column should populate the amount from document id=D + only -ve amounts  from document ids B and C.

I am creating a measure variable for each document Id(i.e. [amount] if [doc id]="a" etc) and using these in the main measure =[Amt] Where ([Doc ID ] ="D" ) +  ([v_b_amount] <0), the amounts are not populating correctly.

Thanks in advance.