Summing an IIF Expression

I am using report builder 3.0 to build a report and have and IIF expression, I am having problems summing this expression if someone could please assist?

The IIF Formula is as follows:

=IIf((Fields!TotalInstalmentDue.Value+Fields!PenaltyInterest.Value-Fields!AmountPaid.Value + Fields!PenaltyReversed.Value) <= 0, 0,   (Fields!TotalInstalmentDue.Value+Fields!PenaltyInterest.Value-Fields!AmountPaid.Value + Fields!PenaltyReversed.Value))

Thanks in advance

Sharon