To get a Running Count or Serial No

Migrateduser

Hi,

I am having two group row and one detail row as follows:

Group 1 : Milestone
Group 2 : Activity
Detail Row: Tasks

But all together i need a running count or serial no for all the rows.
like
1 -milestone1
2 --activity 1
3 ---task1
4 ---task2
5 -milestone 2
6 --activity 2
7 ---task 2

Dose, any one have any idea how to create this aggression in the BIRT report?
A common running count whether it is group row or a detailed row ... dosent matter
serial number is common