#include <bits/stdc++.h>
using namespace std;
template <typename A, typename B>
string to_string(pair<A, B> p);
string to_string(const string&);
string to_string(const char* s);
string to_string(bool b);
template <typename A>
string to_string(A v);
template <typename A, typename B>
string to_string(pair<A, B> p);
void debug_out();
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T);
vector<string> vec_splitter(string s);
void debug_var(vector<string> __attribute__((unused)) args,
               __attribute__((unused)) long long idx,
               __attribute__((unused)) long long LINE_NUM) {
  cerr << endl;
}
template <typename Head, typename... Tail>
void debug_var(vector<string>, long long, long long, Head, Tail...);
const long long mod = 1e9 + 7;
const long long N = 1e6 + 5;
void solve() {
  long double r;
  cin >> r;
  long double d = 7;
  long double pi = 22 / d;
  cout << fixed << setprecision(5);
  cout << (2 * pi * r) << '\n';
}
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cout.tie(NULL);
  solve();
  return 0;
}
string to_string(const string& s) { return '"' + s + '"'; }
string to_string(const char* s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "T" : "F"); }
template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto& x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <typename Head, typename... Tail>
void debug_var(vector<string> args, long long idx, long long LINE_NUM, Head H,
               Tail... T) {
  if (idx > 0)
    cerr << ", ";
  else
    cerr << "Line(" << LINE_NUM << "): ";
  stringstream ss;
  ss << H;
  cerr << args[idx] << " = " << ss.str();
  debug_var(args, idx + 1, LINE_NUM, T...);
}
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}
vector<string> vec_splitter(string s) {
  s += ',';
  vector<string> res;
  while (!s.empty()) {
    res.push_back(s.substr(0, s.find(',')));
    s = s.substr(s.find(',') + 1);
  }
  return res;
}
