I have designed a report to show a Project wise Profit and Loss details as shown in the below screenshot
My challenge is to find an expression that could sum the previous group`s (PNLGroup) total together and show it as the Total at the end of each group

Group a,b,c,d
Current Expression:- (Yello Highlighted Field)
=SWITCH
(
Fields!Calculated_PNLGroup.Value = "a",iif(IsNothing(Sum(Fields!Calculated_NetAmount.Value)),0,Sum(Fields!Calculated_NetAmount.Value)),
Fields!Calculated_PNLGroup.Value = "b", Previous( iif(IsNothing(Sum(Fields!Calculated_NetAmount.Value)),0,Sum(Fields!Calculated_NetAmount.Value))) + iif(IsNothing(Sum(Fields!Calculated_NetAmount.Value)),0,Sum(Fields!Calculated_NetAmount.Value)),
Fields!Calculated_PNLGroup.Value = "c",Previous( iif(IsNothing(Sum(Fields!Calculated_NetAmount.Value)),0,Sum(Fields!Calculated_NetAmount.Value))) + iif(IsNothing(Sum(Fields!Calculated_NetAmount.Value)),0,Sum(Fields!Calculated_NetAmount.Value)),
Fields!Calculated_PNLGroup.Value = "d", Previous( iif(IsNothing(Sum(Fields!Calculated_NetAmount.Value)),0,Sum(Fields!Calculated_NetAmount.Value))) + iif(IsNothing(Sum(Fields!Calculated_NetAmount.Value)),0,Sum(Fields!Calculated_NetAmount.Value))
)
The Output is shown below (Yellow Highlighted)

The total of group a & b is correct according to the expression but Group C (Net Profit) total is wrong
The expected output should be (Group B Total + Sum(Groupc))
Group B Total = -41,893,316.92 + (2,341,630 + 1,331,053 + 780,193 + 231,123) = -37,209,317 instead its showing the value as 12,999,855.77
Kindly advise the correct expression to be used