how to add row number base on group in ssrs report

Question

i am trying to add new column name row number as Sr.No in my ssrs report but number is not coming in proper way .

Image

i have used =RunningValue(Fields!Party.Value,CountDistinct,"DrCr") this formula for row number "DrCr" means Debit/Credit Group . I have also used function :-

Public rn as integer
  Public Function GetRn() AS Integer
   rn = rn +1
    return rn
  End Function

=Code.GetRn

but it give result like

	header
Group	1
	2
Group	3
	4

I want groupwise number

	header
Group	1
	2
Group	1
	2