The Wonder Island Intelligence Service has 16 spies in Tartu. Each of them watches on some of his colleagues. It is known that if spy A watches on spy B, then B does not watch on A. Moreover, any 10 spies can numbered in such a way that the first spy watches on the second, the second watches on the third and so on until the tenth watches on the first. Prove that any 11 spies can also be numbered is a similar manner. combinatorics unsolvedcombinatorics