rownumber(nothing) - skip lines

Question

I'm using rownumber(nothing) to create numbering on my report.
I have subheadings for GROUP AVERAGE inside my dataset. Where I use a background expression to set all rows above GROUP AVERAGE Green, below Red, and the line containing GROUP AVERAGE I leave white.

=IIF(Fields!Dealer.Value = "GROUP AVERAGE","",RowNumber(Nothing))

My problem is that Rownumber is not skipping the GROUP AVERAGE line, and I have a situation like this

1  Store A        80%  
2  Store B        78%  
3  Store C        60%  
   ### GROUP AVERAGE  50% ###  
5  Store D        40%  
6  Store E        30%   
...

I want Store D to be number 4, not 5.

Answer 1

runningvalue(Fields!YOURVALUE.Value,count,nothing)