Sum values from the previous month that are not in next (Churn)

[jvargasmayorga]

Hello Community. I hope you can help me with this:

I have a table where I display the number of vehicle plates per month. What I am trying now is the calculation of the "Churn", that is, the number of vehicles from the previous month that are not found in the following month. For example.

Cant. PLACAS, shows the number of vehicles that exist in each month.
Churn Plate. For example for the month of "May", I need to consult and show how many vehicles from "April" are NOT in the month of "May"

I was first trying to show, in each month, the vehicles of the previous month. Using this formula, but it shows me 0.

sum( {$<NumMesAnio_transito={'$(=num(Max(NumMesAnio_transito)-1, '00'))'}>}aggr(only(Cantidad_transito),PlacaVehiculo,[Año transito], Mes_transito))

I don't know if it is the correct way: First calculate the vehicles of the previous month and then count those that do not exist in the following month. Or is there another way to do it?

Best Regards.

Jose

[jvargasmayorga]

Alguna idea?...