#include <bits/stdc++.h>
using intl = long long;
using itnl = long long;
using itn = int;
using ld = long double;
using namespace std;
const intl INF = 1e9;
const intl MOD = 1e9 + 7;
const ld eps = 1.0e-14;
const ld PI = 3.141592653589;
void fp(bool f) { cout << (f ? "Yes" : "No") << endl; }
void fp(bool f, string s, string t) { cout << (f ? s : t) << endl; }
intl kai(intl k) {
  intl a = 1;
  for (int i = 2; i <= k; i++) {
    a *= i;
  }
  return a;
}
intl gcd(intl a, intl b) {
  if (!b) return a;
  return gcd(b, a % b);
}
intl lcm(intl a, intl b) { return a / gcd(a, b) * b; }
intl digit10(intl a) {
  intl b = 0;
  do {
    a /= 10;
    b++;
  } while (a);
  return b;
}
signed main() {
  cout << fixed << setprecision(10);
  intl n;
  cin >> n;
  cout << 2 * n * PI << endl;
  return 0;
}
