How to implement a calculator of this type in JS?

Question

JavaScript

I wish you good health, colleagues!
I'm not strong in JS, especially in calculators) But fortune doesn't smile at me very much - I need to make up two calculators .. There are no questions about styling, but how to make the calculator work?

Calculators look like this.
I am redesigning the site at the moment, exactly the same is on the old psk-svyatogor.ru/onlinekalkulyator (only with a different styling).
Please help with a script / link / lesson: any material will be worth its weight in gold to me)