from math import *
x = eval(input())

def s(x):
    return f(x)#f(x)是一个多项式方程
while True:
    if s(x) < 0 and s(x+0.05) > 0:
        a,b = x,x+0.05
        while abs(s((a+b)/2)) > 0.0000000000001:
            if s((a+b)/2) < 0:
                a = (a+b)/2
            else:
                b = (a+b)/2
        print((a+b)/2)
        break
    elif s(x) > 0 and s(x+0.05) < 0:
        b,a = x,x+0.05
        while abs(s((a+b)/2)) > 0.0000000000001:
            if s((a+b)/2) < 0:
                a = (a+b)/2
            else:
                b = (a+b)/2
        print((a+b)/2)
        break
    x += 0.05