Kevin is considering going to a rock concert. The ticket will cost him $56, and he will need to spend $3 on gas to drive to the venue and $8 on parking. It will take him 3 hours to get there, attend the concert, and come back. If Kevin did not go to the concert, he would work as a waiter at a nearby restaurant and earn $15 per hour. What is Kevin's opportunity cost of attending the concert? (Enter a whole number, e.g., 123.) $