Dr. Kai protects his computer system from those pesky Cortex spys by incorporating a secret code that only he knows how to work out. Two numbers appear on his screen, and he can't log in until he types in a third number, related to the first two. However, certain Cortex spies kept an eye on him as he entered the secret code several times. For instance, when 1 and 2 appeared on the screen, he typed in 9 to log in. When 3 and 4 appeared, he typed in 20. When 5 and 6 appeared, he typed in 12. Finally, when it was safe to do so, a Cortex spy broke in and started up the computer. The numbers 17 and 12 appeared on the screen. What number should the Cortex spy type in to gain access to Dr. Kai's computer?