P is hot in pursuit of his enemy Q, who is hiding in one of 17 caves. The caves form a linear array, and every night Q moves from the cave he is in to one of the caves on either side of it. P can search two caves each day, with no restrictions on his choice. For example, if P searches (1, 2), (2, 3), ..., (16, 17), then he is certain to catch Q, though it might take him 16 days. What is the shortest time in which P can be guaranteed of catching Q?