Directives in sass. What and how?

Question

Sass

I recently started learning sass. With simple things like nesting, template classes and mixins, it seems clear, but I want something more. So I continued to study on directives. In general, it's difficult. It would be great to have a person who understands this and is ready to help with instructions, but there is no such thing. Therefore, you have to try to "chemize" yourself, because. tutorials about directives have too simple examples. At the moment I decided to try something and I don’t even know how to approach this problem.
Actually, the task: how to make each subsequent ".box" class have a width of + 50px from the previous value, while subtracting 2px from the new value in the same style.
Like, the first .box is 50px wide, the second 98px(+50px -2px), the third 146px and so on, up to a certain value/number of repetitions, whatever.