def sqrtb(n):
    if n<0: raise ValueError('n>=0')
    left,right,x=0,n,n/2
    while not -1e-15<x*x-n<1e-15:
        if x*x>n:
            right,x = x,left+(x-left)/2
        else:
            left,x = x,right-(right-x)/2
    return x

def sqrtB(x):
    if x==0: return 0
    #y,x=x,round(x)
    left,right,ret = 1,x,0
    while left<=right:
        mid = left + (right-left)//2
        if mid<x/mid:
            left = mid+1
            ret = mid
        elif mid==x/mid:
            ret = mid
            break
        else:
            right = mid-1
    return ret

def sqrtn(n):
    if n<0: raise ValueError('n>=0')
    x = n/2
    while not -1e-15<x*x-n<1e-15:
        x = (x+n/x)/2
    return x

def sqrt2(n):
    x = n
    while x*x>n:
        x = (x+n/x)/2
    return x

def sqrt(n):
    x = n
    while x*x>n:
        y,x = x,(x+n/x)/2
        if y==x: break
    return x

def cubeN(n):
    x,y = n/3,0
    while not -1e-15<x-y<1e-15:
        y,x = x,(2/3)*x+n/(3*x*x)
    return x
'''
>>> cubeN(27)
3.0
>>> cubeN(9)
2.080083823051904
>>>
'''