Help writing a formula

I have these 3 columns:

1. Baseline Finish (date column)

2. Actual Finish (date column)

3. Days Late (number column)

I want to write a formula with the following logic:

If [Actual Finish] is null, set [Days Late] to 0

If [Actual Finish] is NOT null, subtract [Baseline Finish] - [Actual Finish] and enter the result to [Dates Late]

How can I write this code?