# D
n = int(input())
i = 1
m = 1
sum = 0
flag = '+'
while i <= n:
if flag == '+':
sum += 1 / m
flag = '-'
elif flag == '-':
sum -= 1 / m
flag = '+'
m += 2
i += 1
# PI / 4 = sum
PI = sum * 4
print('%.10f' %(PI))
