#include <bits/stdc++.h>
using namespace std;
const int INF = 1001001000;
const int mINF = -1001001000;
const long long LINF = 1010010010010010000;
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
const long double PI = 3.141592;
int main() {
  long double r;
  cin >> r;
  cout << fixed << setprecision(10);
  cout << 2 * r * PI << endl;
}
