import sys
import math
from collections import deque
input = lambda: sys.stdin.readline().rstrip()
Yes = lambda boolean: print('Yes') if boolean else print('No')
YES = lambda boolean: print('YES') if boolean else print('NO')
aint = lambda: int(input())
ints = lambda: list(map(int, input().split()))
is_even = lambda x: True if x%2==0 else False
zeros2d = lambda r,c: [[0 for col in range(c)] for row in range(r)]

R = aint()
print(2*R*math.pi())