9. (12 points) The teacher draws 9 cards from 13 cards labeled $1 \sim 13$, and sticks them on the foreheads of 9 students. Everyone can see the numbers of the other 8 people but not their own. (All 9 students are honest and smart, and the cards $6, 9$ cannot be inverted.) The teacher asks: "Now, those who know the number of divisors of their number, please raise your hands." Two people raise their hands. After putting their hands down, three people have the following conversation.
甲: “I know what my number is.”
乙: “Although I don't know what my number is, I already know its parity.”
丙: “My number is 2 less than 乙's and 1 more than 甲's.”
So, the sum of the numbers on the four cards that were not drawn is $\qquad$
【Answer】Solution: The number of divisors is 1 for 1;
The number of divisors is 2 for $2, 3, 5, 7, 11, 13$;
The number of divisors is 3 for $4, 9$;
The number of divisors is 4 for $6, 8, 10$; The number of divisors is 6 for 12.
Each person can only see the numbers on the foreheads of the other 8 classmates, and to determine the number of divisors of their own number, they must see 8 numbers. This can only be the case if they have seen all numbers with 1, 3, 4, and 6 divisors. Therefore, the four cards that were not drawn must have 2 divisors, meaning they are prime numbers. The numbers with a number of divisors not equal to 2 are 7, so 7 people did not raise their hands, meaning the two people who raised their hands have prime numbers on their foreheads. After putting their hands down, A says: "I know what my number is." So the number on A's forehead is not a prime number. B says: "Although I don't know what my number is, I already know its parity." B is still unsure of his number, so he must have a number with 2 divisors, meaning the number on his forehead is a prime number. Knowing the parity, he must have seen a 2 on someone else's forehead, so B's number, other than 2, must be a prime number and thus odd. Since C is 2 less than B, C must also be odd. Since C knows his number, his number cannot be a prime number, so C's number can only be 1 or 9. Since C is 1 more than A, C must be 9, A must be 8, and B must be 11.

Therefore, the prime numbers that appear are 2 and 11.
So the four cards that were not drawn are $3, 5, 7, 13$, and their sum is $3+5+7+13=28$. The answer is: 28.