def str_base(number, base):
    if (number < 0):
        return ('-' + str_base((- number), base))
    (d, m) = divmod(number, base)
    if (d > 0):
        return (str_base(d, base) + digit_to_char(m))
    return digit_to_char(m).lower()
def str_base(number, base):
    if (number < 0):
        return ('-' + str_base((- number), base))
    (d, m) = divmod(number, base)
    if (d > 0):
        return (str_base(d, base) + digit_to_char(m))
    return digit_to_char(m).lower()