11. Guess the number. Sixth-graders Petya, Vasya, Kolya, and Anton made the following statements. Petya: “The number is prime.” Vasya: “The number is 9.” Kolya: “The number is even.” Anton: “The number is 15.” It is known that only one of Petya's and Vasya's statements is true, just as only one of Kolya's and Anton's statements is true. What number is guessed?