def Beta(m):
    """
    Double factorial of odd numbers: a(m) = (2*m-1)!! = 1*3*5*...*(2*m-1)
    :param m: integer
    :return: Integer
    """
    if m <= 0:
        ans = 1
    else:
        ans = 1
        for i in range(1, m):
            ans *= 2 * i + 1
    return ans
