Answer should be an interger
Double Factorial is defined as the product of all integers from 1 up to n that have the same parity (odd or even).
if x is even, then
x!! = x(x-2)(x-4)(x-6)...........4.2
if x is odd, then
x!! = x(x-2)(x-4)(x-6)...........3.1
if x = -1 or 0, then
x!! = 1 from...