double factorial (math.) with "m!!!" denoted by "m!!!". When m is a Nature number, it means the product of all positive integers up to m and and m have the same parity.

Example:
	3!!=1*3=3
	5!!=1*3*5=15
	6!!=2*4*6=48
	8!!=2*4*6*8=384
	Another:
	0!!=1!!=1