#include <bits/stdc++.h>
using namespace std;
template <class T>
using VV = vector<vector<T>>;
template <class T>
inline T SMIN(T& a, const T b) {
  return a = (a > b) ? b : a;
}
template <class T>
inline T SMAX(T& a, const T b) {
  return a = (a < b) ? b : a;
}
template <class Iter>
void __kumaerrc(Iter begin, Iter end) {
  for (; begin != end; ++begin) {
    cerr << *begin << ',';
  }
  cerr << endl;
}
void __kumaerr(istream_iterator<string> it) {
  (void)it;
  cerr << endl;
}
template <typename T, typename... Args>
void __kumaerr(istream_iterator<string> it, T a, Args... args) {
  cerr << *it << "=" << a << ", ", __kumaerr(++it, args...);
}
template <typename S, typename T>
std::ostream& operator<<(std::ostream& _os, const std::pair<S, T>& _p) {
  return _os << "{" << _p.first << ',' << _p.second << "}";
}
const double PI = acos(-1);
int R;
void solve() { cout << 2.0 * PI * R << endl; }
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout << setprecision(12) << fixed;
  cin >> R;
  solve();
  return 0;
}
