#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using VI = vector<int>;
using VL = vector<ll>;
using VS = vector<string>;
using VVI = vector<VI>;
using VVL = vector<VL>;
using PII = std::pair<int, int>;
using PLL = std::pair<ll, ll>;
const int mod = 1e9 + 7, mod2 = 998244353, mod3 = 1e9 + 9;
const ll inf = 1e18;
const string wsp = " ";
const string tb = "\t";
const string rt = "\n";
template <typename T>
void show1dvec(const vector<T> &v) {
  if (v.size() == 0) return;
  int n = v.size() - 1;
  for (int i = 0; i < (int)(n); i++) cout << v[i] << wsp;
  cout << v[n] << rt;
  return;
}
template <typename T>
void show2dvec(const vector<vector<T>> &v) {
  int n = v.size();
  for (int i = 0; i < (int)(n); i++) show1dvec(v[i]);
}
template <typename T, typename S>
void show1dpair(const vector<pair<T, S>> &v) {
  int n = v.size();
  for (int i = 0; i < (int)(n); i++)
    cout << v[i].first << wsp << v[i].second << rt;
  return;
}
template <typename T, typename S>
void pairzip(const vector<pair<T, S>> &v, vector<T> &t, vector<T> &s) {
  int n = v.size();
  for (int i = 0; i < (int)(n); i++) {
    t.push_back(v[i].first);
    s.push_back(v[i].second);
  }
  return;
}
template <typename T>
void maxvec(vector<T> &v) {
  T s = v[0];
  int n = v.size();
  for (int i = 0; i < (int)(n - 1); i++) {
    if (s > v[i + 1]) {
      v[i + 1] = s;
    }
    s = v[i + 1];
  }
}
template <typename T, typename S>
bool myfind(T t, S s) {
  return find(t.cbegin(), t.cend(), s) != t.cend();
}
bool check(int y, int x, int h, int w) {
  return 0 <= y && y < h && 0 <= x && x < w;
}
VL cumsum(const VL &v) {
  int s = 0;
  VL ret = {0};
  for (int i = 0; i < (int)(v.size()); i++) {
    s += v[i];
    ret.push_back(s);
  }
  return ret;
}
bool iskadomatsu(int a, int b, int c) {
  return (a != b && b != c && c != a) && ((a > b && b < c) || (a < b && b > c));
}
VS split(string s, char c) {
  VS ret;
  string part;
  s += c;
  for (int i = 0; i < (int)(s.length()); i++) {
    if (s[i] == c) {
      ret.emplace_back(part);
      part = "";
    } else if (s[i] != c) {
      part += s[i];
    }
  }
  return ret;
}
int main() {
  cout << "DEBUG MODE" << endl;
  ifstream in("input.txt");
  cin.rdbuf(in.rdbuf());
  double r;
  cin >> r;
  cout << 6.28318530717 * r << rt;
  return 0;
}
