from math import sqrt

N = int(input())


def func(x):
    return sqrt(x*x*x+N*N*N)


S = 0
n = 1 << 20
for i in range(n+1):
    S += (1 if i == 0 or i == n else 4 if i % 2 else 2) * func(i/n)
print(S / (3*n))