// -----------------------------------
// Author     : MatsuTaku
// Country    : Japan
// Created    : 04/19/20 20:43:13
// -----------------------------------

#include <bits/stdc++.h>

using namespace std;
using ll = long long;

int main() {
  cin.tie(nullptr); ios::sync_with_stdio(false);

  double r; cin>>r;
  cout <<fixed<< r*2*M_PI << endl;

  return 0;
}
