#include <bits/stdc++.h>
using namespace std;
template <class T>
using vec = vector<T>;
template <class T>
using vvec = vector<vec<T>>;
template <typename T1, typename T2>
ostream& operator<<(ostream& os, const pair<T1, T2>& p) {
  os << "{" << p.first << ", " << p.second << "}";
  return os;
}
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
const int INF = (long long)1e9;
const long long INFLL = (long long)1e18 + 1;
const long long MOD = (long long)1e9 + 7;
const double PI = acos(-1.0);
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(20);
  double r;
  cin >> r;
  cout << r * 2 * PI << endl;
}
