sum of calculated expression in rdlc report

I want sum of salary column:

this is expr for salary column:

=iif(fields!Items.Value="A",(cdec(sum(fields!sumofincome.Value)/1000*3)), iif(fields!Items.Value="B",(cdec(sum(fields!sumofincome.Value)/1000*1.5)),iif(fields!Items.Value="C",(cdec(sum(fields!sumofincome.Value)/1000*0.5)),0)))`