double factorial calculator

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

number:
   
double factorial:
   

double multiplication 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.

Example：
	3!!=1*3=3
	5!!=1*3*5=15
	6!!=2*4*6=48
	8!!=2*4*6*8=384
	another0!!=1!!=1