Double Factorial Calculator

illustrate: When entering numbers, separate them with commas (,), spaces, tabs, or separate lines.

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