#include <bits/stdc++.h>
using namespace std;
template <typename T>
void VARS(T a) {
  cout << a << "\n";
}
template <typename T, typename... Types>
void VARS(T a, Types... bs) {
  cout << a << " ";
  VARS(bs...);
}
template <typename T>
istream& operator>>(istream& is, vector<T>& the_vector) {
  for (int i = 0; i < (int)the_vector.size(); i++) {
    is >> the_vector[i];
  }
  return is;
}
template <typename T>
ostream& operator<<(ostream& os, vector<T>& the_vector) {
  for (int i = 0; i < (int)the_vector.size(); i++) {
    os << the_vector[i] << (i + 1 == (int)the_vector.size() ? "" : " ");
  }
  return os;
}
template <typename T>
string join(vector<T>& the_vector, string mid = " ") {
  stringstream ss;
  for (int i = 0; i < (int)the_vector.size(); i++) {
    ss << the_vector[i] << (i + 1 == (int)the_vector.size() ? "" : mid);
  }
  return ss.str();
}
int main() {
  int n;
  cin >> n;
  cout
      << (double)2 * n *
             3.1415926535897932384626433832795028841971693993751058209749445923078164
      << "\n";
  return 0;
}
