How to do a WHERE clause in =IIF(ISNOTHING(Sum(Fields!Asset.Value)),"-",Sum(Fields!Asset.Value))

I have this code

=IIF(ISNOTHING(Sum(Fields!Asset.Value)),"-",Sum(Fields!Asset.Value))

but I want it to do it where the Group=1

How do I change the line of code above to only sum the rows where the Group=1?

I tried this but it doesn't work:

=IIF(ISNOTHING(Sum(Fields!Asset.Value) WHERE Group=1),"-",Sum(Fields!Asset.Value) WHERE Group=1)

I got this to work but I really don't think it is accurate:

lcerni