import math
import numpy as np
from matplotlib import mlab
import pylab
factorial = 1.0
sign = 1.0
x = 0.0
result = 0.0
x = math.pi/2
n = 1
n = input ()
slave = 1.0
fact = 1.0
term = x
for i in range (1,n):
    result = result + term
    sign = sign * (-1)
    slave = slave + 2.0
    fact = fact * (slave - 1) * slave
    term = term * x * x * sign / (1.0*fact)
print result