#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
const int INF = 1e9;
const int N = 1e6 + 6;
long long powmod(long long a, long long b, long long mod) {
  long long res = 1;
  a %= mod;
  for (; b; b >>= 1) {
    if (b & 1) res = (res * a) % mod;
    a = (a * a) % mod;
  }
  return res;
}
long long gcd(long long a, long long b) {
  while (b > 0) {
    long long t = a % b;
    a = b, b = t;
  }
  return a;
}
long long lcm(long long a, long long b) { return (a / gcd(a, b)) * b; }
long long is_prime(long long n) {
  if (n <= 1 || n > 3 && (n % 2 == 0 || n % 3 == 0)) return 0;
  for (long long i = 5, t = 2; i * i <= n; i += t, t = 6 - t)
    if (n % i == 0) return 0;
  return 1;
}
long long modI(long long a, long long m) {
  long long m0 = m, y = 0, x = 1;
  if (m == 1) return 0;
  while (a > 1) {
    long long q = a / m;
    long long t = m;
    m = a % m;
    a = t;
    t = y;
    y = x - q * y;
    x = t;
  }
  if (x < 0) x += m0;
  return x;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  double r;
  cin >> r;
  cout << fixed << setprecision(8) << 2 * 3.141592653589793 * r;
  return 0;
}
