def double_factorial(input_num): 
    """
  Calculate the double factorial of specified number
  
    >>>double_factorial(5)
    15
    >>>double_factorial(8)
    384
    >>>double_factorial(3)
    3
    >>>double_factorial(0)
    1
    >>>-1
    Traceback (most recent call last):
      File "<string>", line 11, in <module>
      File "<string>", line 4, in doublefactorial
    ValueError: double_factorial() not defined for negative values
    """
    if (input_num<0):
       raise ValueError("double_factorial() not defined for negative values")
    if (input_num == 0 or input_num == 1): 
        return 1; 
    return input_num * double_factorial(input_num - 2)