#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i <= (n); i++)
#define ALL(x) (x).begin(), (x).end()
#define SIZE(x) ((int)(x).size())
#define MAX(x) *max_element(ALL(x))
#define INF 1000000000000 //10^12
#define MOD 10000007      //10^9+7
#define root sqrt
using namespace std;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vs = vector<string>;
using ll = long long;
// vector入力
template <typename T>
istream &operator>>(istream &is, vector<T> &vec)
{
  for (T &x : vec)
    is >> x;
  return is;
}
// vector出力
template <typename T>
ostream &operator<<(ostream &os, vector<T> &vec)
{
  // os << '{';
  for (int i = 0; i < vec.size(); i++)
  {
    os << vec[i] << (i + 1 == vec.size() ? "" : ' ');
  }
  // os << '}';
  return os;
}

int main()
{
  int r;
  cin >> r;
  double ans = 3.141592648777698869248;
  printf("%.10lf\n", r * 2 * ans);
}