#include <bits/stdc++.h>
template <class T>
inline void sort(T& t) {
  std::sort((t).begin(), (t).end());
}
template <class T>
inline void rsort(T& t) {
  std::sort((t).rbegin(), (t).rend());
}
template <class T>
inline void unique(T& t) {
  (t).erase(unique((t).begin(), (t).end()), (t).end());
}
template <class T, class S>
inline bool chmax(T& t, const S& s) {
  if (s > t) {
    t = s;
    return true;
  }
  return false;
}
template <class T, class S>
inline bool chmaxE(T& t, const S& s) {
  if (s >= t) {
    t = s;
    return true;
  }
  return false;
}
template <class T, class S>
inline bool chmin(T& t, const S& s) {
  if (s < t) {
    t = s;
    return true;
  }
  return false;
}
inline void BR() { std::cout << "\n"; }
using ll = long long;
using PAIR = std::pair<ll, ll>;
using VS = std::vector<std::string>;
using VL = std::vector<long long>;
using VVL = std::vector<VL>;
using VVVL = std::vector<VVL>;
using VD = std::vector<double>;
template <class T>
using V = std::vector<T>;
using std::cout;
constexpr char endl = '\n';
using std::bitset;
using std::cin;
using std::list;
using std::map;
using std::multimap;
using std::multiset;
using std::pair;
using std::priority_queue;
using std::queue;
using std::set;
using std::stack;
using std::string;
using std::tuple;
using std::unordered_map;
using std::unordered_multimap;
using std::unordered_multiset;
using std::unordered_set;
using std::vector;
struct Preprocessing {
  Preprocessing() {
    std::cin.tie(0);
    std::ios::sync_with_stdio(0);
  };
} _Preprocessing;
inline signed pow(signed, signed) {
  assert(false);
  return -1;
}
namespace std {
template <>
class hash<std::pair<ll, ll>> {
 public:
  size_t operator()(const std::pair<ll, ll>& x) const {
    return hash<ll>()(1000000000 * x.first + x.second);
  }
};
}  // namespace std
template <class T>
std::istream& operator>>(std::istream& is, vector<T>& vec) {
  for (T& x : vec) is >> x;
  return is;
}
constexpr ll MOD = 1000000007;
signed main() {
  double n;
  cin >> n;
  double pi = 3.1415926535;
  cout << std::fixed << std::setprecision(10) << 2 * n * pi << endl;
}
