#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using vi = vector<int>;
using vii = vector<vector<int>>;
using vl = vector<ll>;
using vll = vector<vector<ll>>;
using vlll = vector<vector<vector<ll>>>;
using vc = vector<char>;
using vcc = vector<vector<char>>;
using pi = pair<int, int>;
using pl = pair<ll, ll>;
ll mod = ll(1e9) + 7;
ll inf = ll(1e18);
void bl(bool b, ostream& out = cout) { out << (b ? "Yes" : "No") << endl; }
void bl2(bool b, ostream& out = cout) { out << (b ? "YES" : "NO") << endl; }
template <typename T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
int dx[] = {1, 0, -1, 0, 1, -1, 1, -1};
int dy[] = {0, 1, 0, -1, 1, -1, -1, 1};
using namespace std;
namespace dbg {
using ll = long long;
class Debug {
 public:
  Debug() {}
  template <typename T>
  void vd(vector<T> vec);
  template <typename T>
  void vd2(vector<vector<T>> vec);
  template <typename T>
  void ad(T* arr, ll n);
  template <typename T, size_t N>
  void ad2(T (&arr)[N], ll n);
  template <typename T>
  void md(T mp);
};
template <typename T>
void Debug::vd(vector<T> vec) {
  for (auto vv : vec) cout << vv << " ";
  cout << endl;
}
template <typename T>
void Debug::vd2(vector<vector<T>> vec) {
  for (auto vv : vec) {
    for (auto vvv : vv) cout << vvv << " ";
    cout << endl;
  }
}
template <typename T>
void Debug::ad(T* arr, ll n) {
  for (int i = 0; i <= n; i++) cout << arr[i] << " ";
  cout << endl;
}
template <typename T, size_t N>
void Debug::ad2(T (&arr)[N], ll n) {
  for (int i = 0; i <= N; i++)
    for (int j = 0; j <= n; j++) cout << arr[i][j] << " ";
  cout << endl;
}
template <typename T>
void Debug::md(T mp) {
  cout << "key : value" << endl;
  for (auto const& m : mp) cout << m.first << " : " << m.second << endl;
  cout << endl;
}
}  // namespace dbg
using dbg::Debug;
Debug debug;
class ACirclePond {
 public:
  void solve(std::istream& in, std::ostream& out) {
    double r;
    in >> r;
    out << fixed << setprecision(10) << 2 * M_PI * r << "\n";
  }
};
int main() {
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  ACirclePond solver;
  std::istream& in(std::cin);
  std::ostream& out(std::cout);
  solver.solve(in, out);
  return 0;
}
