#include <bits/stdc++.h>
using namespace std;
const long long int mod = 1e9 + 7;
const long long int inf = 1e18 + 1e17;
long long int modInverse(long long int a, long long int m) {
  long long int m0 = m;
  long long int y = 0, x = 1;
  if (m == 1) return 0;
  while (a > 1) {
    long long int q = a / m;
    long long int t = m;
    m = a % m, a = t;
    t = y;
    y = x - q * y;
    x = t;
  }
  if (x < 0) x += m0;
  return x;
}
long long int powm(long long int a, long long int b) {
  a = a % mod;
  long long int res = 1;
  while (b) {
    if (b & 1) res = (res * a) % mod;
    a = (a * a) % mod;
    b >>= 1;
  }
  return (res % mod);
}
const long long int N = 1e5 + 6;
const long double pi = 3.14159265358979323;
void solve() {
  long double r;
  cin >> r;
  r = 2 * pi * r;
  cout << setprecision(6) << fixed;
  cout << r << '\n';
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long int t = 1;
  for (int i = 1; i <= t; i++) {
    solve();
  }
}
