#include <bits/stdc++.h>
using namespace std;
using ll = long long int;
using ull = unsigned long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
template <typename T, typename S>
inline ostream& operator<<(ostream& os, const pair<T, S> p) {
  cout << "[" << p.first << ";" << p.second << "]";
  return os;
}
template <typename T>
inline ostream& operator<<(ostream& os, const vector<T>& v) {
  for (auto el : v) cout << el << " ";
  return os;
}
template <typename T>
inline ostream& operator<<(ostream& os, const set<T>& v) {
  for (auto el : v) cout << el << " ";
  return os;
}
template <typename T, typename S>
inline ostream& operator<<(ostream& os, const map<T, S>& v) {
  for (auto el : v) cout << el << " ";
  return os;
}
template <typename T>
inline ostream& operator<<(ostream& os, const multiset<T>& v) {
  for (auto el : v) cout << el << " ";
  return os;
}
namespace ___debug {
struct DebugStream {
 private:
  bool is_first;

 public:
  DebugStream(bool _is_first) : is_first(_is_first) {}
  template <typename T>
  DebugStream operator<<(const T& value) const {
    assert(0);
    if (is_first) cout << "[DBG] ";
    cout << value;
    return DebugStream(false);
  };
  template <typename T>
  DebugStream printArray(T* l, T* r) {
    assert(0);
    if (is_first) cout << "[DBG] ";
    while (l != r) {
      cout << (*l);
      ++l;
      if (l == r) {
        cout << '\n';
      } else {
        cout << ' ';
      }
    }
    return DebugStream(false);
  }
};
DebugStream instance(true);
};  // namespace ___debug
const int INT_INF = (int)(2e9);
const ll LL_INF = (ll)(2e18);
const int NIL = -1;
static mt19937 _g(time(nullptr));
inline ll randint(ll a, ll b) {
  ll w = (_g() << 31LL) ^ _g();
  return a + w % (b - a + 1);
}
inline void fast_io() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
};
template <typename T>
inline T sign(T x) {
  return T(x > 0) - T(x < 0);
}
template <typename T>
inline T fetch() {
  T ret;
  cin >> ret;
  return ret;
}
template <typename T>
inline vector<T> fetch_vec(int sz) {
  vector<T> ret(sz);
  for (auto& elem : ret) cin >> elem;
  return ret;
}
void solve() {
  ld v = fetch<ld>();
  cout << fixed << setprecision(12) << (v * 6.28318530717958623200) << '\n';
}
int main() {
  fast_io();
  solve();
  return 0;
}
