Dynamically indexing using DAX

Hi!

I have the next challenge ahead, but I have not been able to solve. I tell you, I want to have a dynamic index that only depends on the records shown in a table. I give you an example:

I have the next table with Countries and cities:

What I would like to have is a measure  which allows creating the following index "#" (I have tried it with a calculated column, but when it is calculated in processing time and not in execution time, it is not possible):

In the event that we filter by "Spain"  it should show the following:

If we look at the first table we have "City = Madrid" with a value in the measure "#" of 5, in the second table the value taken by the "#" is 3, which is correct, since it occupies the position 3 of the table.

In principle, you don't want this index to be conditioned by anything, which I think Rank functions aren't valid.

Does anyone dare to help me? Is it possible to do?

Thanks!

Best Regards