A doctor prescribes 200 mg of a medication on the first day, and the dosage is reduced by half every day, for one week. What is the total amount of medication prescribed?

I know the answer, but the way I got the answer is definitely incorrect. This question was meant to be solved using an arithmetic or geometric equation, and although I know the majority of them, I seem to have forgotten the exact equation for solving a question like this. The correct answer is 397 mg. Can anyone help me?