Sharepoint calculated collumn date difference between 3 dates when one is now

Hi I have a list in the S.Point Office 365 with a number of columns to register and track claims. One of the columns is a calculated one - to catch the duration of claims processing . I would like to calculate it as a difference between dates and shown in days. To do it i want to use 2 other columns:

[RegistrationDate] - date of new registered claim

[EndDate] - finalization of the claim

and one more important info is todays date(now).

.

The logic to be done by the formula should be:

1. check if the [EndDate] is empty

a. If [EndDate] is empty then calculate the difference as:

todays date-[RegistrationDate]

b. If [EndDate] is filled in then calculation should be: [EndDate]-RegistrationDate

I was trying many options but without success.

I'll appreciate your support

Regards Tom