Double factorial

The double factorial of a non-negative integer $n$, denoted by $n!!$, is defined as the product of all integers between 1 and $n$ that have the same parity as $n$:

$n!!=\left\{\begin{array}{l}n·(n-2)·(n-4)\cdots 4·2\text{if n is even}\\ n·(n-2)·(n-4)\cdots 3·1\text{if n is odd}\end{array}\right.$

$0!!=1$ by convention.

Integer!!

Description

Given an integer between 1 and 65535, returns $n!!$.