import math

x = float(input())

def solve(x):
    m = int(x)
    r = x - m
    dif1 = 0
    for n in range(1, m + 1):
        dif1 += 1 / (n**2)
    dif2 = 0
    for n in range(m + 1, 10**6):
        dif2 += (2*n + r)/(n * (n + r))**2
    return (math.pi)**2 / 6 - dif1 - r * dif2

print(solve(x))