Double Factorial Calculator

number:
Double factorial:

Double factorial is represented by "m!!". When m is a natural number, it represents the product of all positive integers not exceeding m and having the same parity as m.

示例:
	3!!=1*3=3
	5!!=1*3*5=15
	6!!=2*4*6=48
	8!!=2*4*6*8=384
	另0!!=1!!=1