It is a well-known fact that Dr. Barnes rides a skateboard, sometimes even on campus. Suppose that Dr. Barnes selects a skateboard by first picking one of two skateboard shops at random and selecting a skateboard from that shop at random. The first shop contains two "rad" skateboards and three "gnarly" skateboards, and the second shop contains four "rad" skateboards and one "gnarly" skateboard. What is the probability that Dr. Barnes picked a skateboard from the first shop if he has selected a "gnarly" skateboard?