def list_num(l):
    """
    list to number

    >>> list_num([1,2,3,4,5,6,7,8,9])
    123456789

    >>> list_num(i for i in range(9, 0, -1))
    987654321
    """
    s = 0
    for n in l:
        s = s * 10 + n
    return s