Sam and Appu are the greatest mathematicians ever born. Both of them have two prime numbers on their foreheads and are told that any three of them form the sides of a triangle with prime perimeter. They take turns stating whether they can deduce the numbers on their own foreheads. Sam sees 13 and 17 on Appu’s forehead and each of them has stated “Don’t know” on each of their first two turns. It is now Sam’s third turn. What are the numbers on Sam’s forehead?