This is an ordinary high school math problem, and the answer should be: n! ( 1- 1/2! + 1/3! - 1/4! + 1/5! ... +(- 1) n- 1 square/! ), most elementary number theory books are recorded. The answer upstairs is wrong, because the problem he is considering is equivalent to a repeated arrangement, and there should be an answer in "100 Elementary Mathematics Title". I have the electronic document of this book and the book itself here.