Adding an expression to an existing expression

I'm using the below expression to update a field by taking the rowid and turning it into a number.  Now I'm trying to tell this expression to only update this field IF the opportunity type is equal to contract/bid.  Does any one know how to add this statement in...everything I've tried doesn't work.  Please help.
Current expression -

Right(RowIdToRowNum([<OpportunityId>]), Len(RowIdToRowNum([<OpportunityId>]))-InStr(RowIdToRowNum([<OpportunityId>]),'-'))

Thanks.