#include<iostream>
#include<stdio.h>
#include <vector>
#include <algorithm>
using namespace std;

int main(){
  //整数の時はint
  int a,b,m;
  double c;
  //文字の時はstring
  //string a;
  const double PI=3.14159265358979323846;
  cin>>c;
  cout<<c*PI*2.0<<endl;

 
  return 0;
}

