Value for a Register Number

Hi, In the Account record type,  I have an integer field named Register, and I need to do the following:
a) If the Parent Account field is null, write in the Register field a 1.
b) If the Parent Account field is not null, write in the Register field a 2.

How do you do the expression function? Does it go in the Predefined Value?

I already tried many functions with the IIF and IFNULL.

Thanks