python 3 解法 ，考虑小于1以及负数的情况

import sys

def cube():
    num = float(sys.stdin.readline().strip())
    if num == 0:
        return 0
    if num > 0:
        sig = 1
    else:
        sig = -1
    num = abs(num)

    if num > 1:
        start = 0
        end = num
    else:
        start = num
        end = 1
    mid = (end + start) / 2
    while abs(mid ** 3 - num) > 0.001:
        if mid ** 3 > num:
            end = mid
        else:
            start = mid
        mid = (end + start) / 2
    print(round(sig * mid, 1))

cube()