# radius
# Fig. 60.2 (Fig 60.3 not implemented)
# inputs: 10, 100, 1000, 10000, ...

import math

n = int(input("n=? "))
r = 2
for i in range(4,n+1):
  r = r/math.cos(math.pi/i)
print(f"r == {r:6f}")