#include <bits/stdc++.h>
using namespace std;
using ll = int64_t;
using ull = uint64_t;
using uint = uint32_t;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pil = pair<int, ll>;
using pil = pair<int, ll>;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
template <typename T>
inline istream& operator>>(istream& i, vector<T>& v) {
  for (int j = 0; j < (v.size()); ++j) i >> v[j];
  return i;
}
template <typename T>
string join(const vector<T>& v) {
  stringstream s;
  for (int i = 0; i < (v.size()); ++i) s << ' ' << v[i];
  return s.str().substr(1);
}
template <typename T>
inline ostream& operator<<(ostream& o, const vector<T>& v) {
  if (v.size()) o << join(v);
  return o;
}
template <typename T1, typename T2>
inline istream& operator>>(istream& i, pair<T1, T2>& v) {
  return i >> v.first >> v.second;
}
template <typename T1, typename T2>
inline ostream& operator<<(ostream& o, const pair<T1, T2>& v) {
  return o << v.first << "," << v.second;
}
constexpr double eps = 1e-6;
constexpr ll LINF = 100200300400500600ll;
constexpr int INF = 2 * ((1 << 30) - 1) + 1;
template <class T>
T chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return 1;
  } else
    return 0;
}
template <class T>
T chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return 1;
  } else
    return 0;
}
constexpr int dx[4] = {0, 1, 0, -1};
constexpr int dy[4] = {-1, 0, 1, 0};
constexpr int mod = 1000000007;
void Main() {
  int r;
  cin >> r;
  cout << setprecision(10) << 2 * r * M_PI << endl;
}
signed main(int argc, char** argv) {
  cin.tie(nullptr);
  std::ios::sync_with_stdio(false);
  ;
  ifstream iofi;
  if (argc > 1) {
    iofi.open(argv[1]);
    std::cin.rdbuf(iofi.rdbuf());
  };
  int loop = 1;
  if (argc > 2) loop = stoi(argv[2]);
  for (int i = 0; i < (loop); ++i) Main();
}
