(2n)!/(2n-3)
(2n)! / (2n -3) =
[2n * (2n-1) *(2n -2) * (2n -3) * ......* 1] / [ (2n-3) ] =
2n * (2n - 1) * (2n -2) * (2n - 4) * (2n - 5) * .... * 3 * 2 * 1