Help with beginner exercise

Create an html page, add 2 text boxes in which the user will input 2 numbers: n and m
Add a button that, on click, will calculate the sum of all even numbers between n and m, regardless if n < m or m < n. The result will be displayed in an alert.”
So far, I have this and this is all the knowledge I have to apply.
Can you please help me correct it?

Put the challenge link to give us clear idea about what you need to know.

I posted above what I did so far, I am not sure what else I could do.