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:

Step Activity 36 Transaction 2 37 38 39 <tbody> </tbody>

<tbody>
</tbody>

What I need is

What I need id

Step 1 for Transaction2 and merged.

Can you please help ?

Thank you.