#include <bits/stdc++.h>
using namespace std;
template <class T>
ostream& operator<<(ostream& os, vector<T> V) {
  os << "[ ";
  for (auto v : V) os << v << " ";
  return os << " ]";
}
template <class T>
ostream& operator<<(ostream& os, set<T> V) {
  os << "[ ";
  for (auto v : V) os << v << " ";
  return os << " ]";
}
template <class T, class R>
ostream& operator<<(ostream& os, map<T, R> V) {
  os << "[ ";
  for (auto v : V) os << "(" << v.first << ":" << v.second << ")";
  return os << " ]";
}
template <class T, class R>
ostream& operator<<(ostream& os, pair<T, R> V) {
  return os << "(" << V.first << "," << V.second << ")";
}
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) {
  cout << name << " : " << arg1 << endl;
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
  const char* comma = strchr(names + 1, ',');
  cout.write(names, comma - names) << " : " << arg1 << " | ";
  __f(comma + 1, args...);
}
clock_t clk = clock();
const long long int Mod = 1e9 + 7;
const long long int MAXN = 1e5;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cout << fixed << setprecision(25);
  long double pi = 6.28318530717958623200 / 2.0;
  long long int r;
  cin >> r;
  cout << 2.0 * pi * r << "\n";
  return 0;
}
