# expdist3 
# Fig. 22.6 
# input: 10000

import math, random

n = int(input("n=? "))
sum = 0
for i in range(n): 
  while True:
    x = random.uniform(-1,1)
    y = random.uniform(-1,1)
    if x*x + y*y <= 1:
      break
  while True:
    z = random.uniform(-1,1)
    u = random.uniform(-1,1)
    if z*z + u*u <= 1:
      break
  dist = math.sqrt((x-z)*(x-z) + (y-u)*(y-u))
  sum += dist
print(f"exp. dist. == {sum/n:.6f}")