#include <bits/stdc++.h>
using namespace std;
template <typename T>
using V = vector<T>;
using VL = V<long long>;
using VVL = V<VL>;
template <typename T, typename U>
using P = pair<T, U>;
using PL = P<long long, long long>;
using VPL = V<PL>;
template <typename T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
struct exit_exception : public std::exception {
  const char* what() const throw() { return "Exited"; }
};
template <typename T>
void drop(T res) {
  cout << res << endl;
  throw exit_exception();
}
const long long INF = 1e18;
void solve();
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  try {
    solve();
  } catch (exit_exception& e) {
  }
  return 0;
}
const long double PI = 3.141592653589793238462643383279;
void solve() {
  long long R;
  cin >> R;
  long double res = 2 * PI * R;
  cout << fixed << setprecision(20) << res << endl;
}
