B'One day, n people ( n is an even number) met on a plaza and made two round dances, each round dance consists of exactly frac{n}{2} people. Your task is to find the number of ways n people can make two round dances if each round dance consists of exactly frac{n}{2} people. Each person should belong to exactly one of these two round dances. Round dance is a dance circle consisting of 1 or more people. Two round dances are indistinguishable (equal) if one can be transformed to another by choosing the first participant. For example, round dances [1, 3, 4, 2] , [4, 2, 1, 3] and [2, 1, 3, 4] are indistinguishable. For example, if n=2 then the number of ways is 1 : one round dance consists of the first person and the second one of the second person. For example, if n=4 then the number of ways is 3 . Possible options: Your task is to find the number of ways n people can make two round dances if each round dance consists of exactly frac{n}{2} people. The input contains one integer n ( 2 <= n <= 20 ), n is an even number. Print one integer -- the number of ways to make two round dances. It is guaranteed that the answer fits in the 64 -bit integer data type.'...