Riddle of the Dreaming Cat

A peculiar cat dreams of one gray and twelve brown mice running clockwise around himself. Being a cat, he plans to eat each and every mouse. Being a peculiar cat he wants to eat them in a pattern. He wants to eat a mouse let two mice run by and then eat a mouse until the last mouse running is the gray mouse and of course he will devour that one last. The question is- which mouse does he start with?