Mathematical Operations with Multiple Dials

I am trying to set up multiple dials that interact with one another in a mathematical formula. Specifically, I have four dials, let's call them A, B, C, and D. I'd like to show that A - B - C = D. I'd like the value of D to change when a learner turns either A, B, or C. Essentially, I'd like to set it up such that for every step that A is turned to the right, D is increased by 1, and for every step that A is turned to the left, D is decreased by 1. (And similarly for B and C, only in the opposite direction.)

I cannot figure out a way to do this. I have tried setting this up such that every time A is turned, A is added to D. However, unless D is reset to 0 every time, it will add A to the existing value of D. For example:

1. Turn A from 0 to 1 --> D is changed from 0 to 1
2. Turn A from 1 to 2 --> D is changed from 1 to 3 (because it is adding 2 to 1)

I also cannot set it up such that every time A is turned, 1 is added to D, because:

1. Turn A from 0 to 1 --> D is changed from 0 to 1
2. Turn A from 1 to 0 --> D is changed from 1 to 2

I know I can have the learner set their preferred levels of all three dials (A, B, and C) and then click on a button to get the result for D. But I'd really like to allow the learner to see immediate changes in D as soon as just one of A, B, or C is turned.

 Can anyone help? Thanks in advance!