I am trying to remember a basic theorem from number theory involving moving along points on a circle with a set number of steps each time. The theorem goes something like this: if you start on a circle with n points and move k points each time, you will eventually land on every point on the circle if and only if n and k are coprime. I know this is a famous basic result, but I can't remember what the name of it was or which general theorem this was a special case of. Pic related is an example.