Increment column based on the change of another column

[BKChedlia]

Dear all,

I need your help to add an incremental column A related to column B, where the index is always incremented by 1 each time the value of column B changes, PS: column B is merged each time we have new Activitiy

This is the current excel:

[TABLE="width: 144"]
<tbody>[TR]
[TD="width: 72"][TABLE="width: 243"]
<tbody>[TR]
[TD]Step[/TD]
[TD]Activity[/TD]
[/TR]
[TR]
[TD]36[/TD]
[TD]Transaction 2[/TD]
[/TR]
[TR]
[TD]37[/TD]
[/TR]
[TR]
[TD]38[/TD]
[/TR]
[TR]
[TD]39[/TD]
[/TR]
</tbody>[/TABLE]
[/TD]
[TD="width: 72"][/TD]
[/TR]
</tbody>[/TABLE]


What I need is

What I need id

Step 1 for Transaction2 and merged.

Can you please help ?

Thank you.