(Java) Not sure of the term, but basically how do I get the "opposite" of a number in a range?

What I mean is, let's say that I have number between 0 - 400. If I were to pass in 0, it would return 400, if I were to pass in 1 it would return 399, and so on and so forth. Is there an equation that will let me do this? (Yes, I know this isn't really java as much as it is plain Math)

EDIT: WOW, I'm and idiot. How did I NOT realize that this was subtraction?

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