import math 

def int(f,a,b,N):
  sum=0.0
  start=float(a)
  step=float(b-a)/N
  for i in range(N):
    sum+= (f(start+step*i)+f(start+step*(i+1)))*step/2
  return sum
    


f=lambda x: math.sqrt(1-x**2)
a=0
b=1
N=input("Number of steps: ")

print int(f,a,b,N)-math.pi/4