Help needed on calculation depending on dropdown list item selected

This is my first post and I am new to this, so I hope someone can assist me.

I require help in incorporating a formula/calculation script into an Adobe PDF form. The aim is to have a dropdown box that, when selected, automatically adds the correct number of points in the adjacent box.

For example, if option A is selected, the adjacent box should display 1 point. If option B is selected, and option C is selected, the adjacent box should display 6 points. Similarly, if option B is selected, and option D is selected, the adjacent box should display 10 points.

In summary:

A = 1

B = 4

C = 6

D = 10

Although this may seem like a simple task, I have not been able to find a straightforward solution during my searches. I've attached a screenshot which may explain it better!

Thank you in advance!