c# how to write abstract method that return enum

I have a base class and few classes derived from it.

I want to declare an enum in the base class as abstract and each class that derived from it will have to create its own different enum.

How can I do it?

I tried the following: declare an abstract method in the base class that return enum type and each class that implement it will return its own enum:

public abstract enum RunAtOptions();

But I get compile error

"The modifier abstract is not valid for this item"