Is my way to right simple JavaScript DOM project is Correct?

I am coding with freecodecamp. I am learning DOM. This is my simple project. Can you please suggest me if this is the correct way to write the project? Can you please suggest me the better way to right?
here is my project.

Thank you very much in advance.

@camperextraordinaire Thank you very much. I change parseInt to parseFloat and it works. Thank you very much.

Thank you very much. I am working on this. After I finished I will post here again. Thank you very much for your suggestions. I want to validate the input without using REGEX for a moment. I am going to do it soon and let you know. Thank you.

I did like if (isNaN(userNumber)), It works, But not sure if this is the standard way to validate without using REGEX .