#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using st = string;
using db = double;
using vll = vector<long long>;
using vdb = vector<db>;
using vvll = vector<vll>;
using vst = vector<st>;
using vchar = vector<char>;
using pll = pair<ll, ll>;
using vpll = vector<pll>;
using vvpll = vector<vpll>;
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
const ll mod = 1000000007;
const ll inf = 1LL << 60;
int main() {
  db r;
  cin >> r;
  cout << fixed << setprecision(3);
  cout << r * 3.141592 * 2 << endl;
}
