An antonym for Inexact is Exact. The prefix "In-" means "not". Other words that are the opposite of inexact are: Accurate, Correct, Right, True, Valid, Actual, Factual, Genuine, Known, Precise, Real,...
If you have to be opposite on opposite day, that makes it a day, and then there was no opposite day. But that can't be true since you HAVE to be opposite, which means that you have to be normal even...