import java.io.*;
import java.util.*;
import java.math.*;
 
class Main
{
    public static void main(String[] args)throws IOException
    {
        BufferedReader br =  new BufferedReader(new InputStreamReader(System.in));
        Scanner sc = new Scanner(System.in);
        int t = 1;
        while(t-->0)
        {
        	   int r = sc.nextInt();
                  double pie=3.141592653589793238;
                  double d = 2.0*pie*r;
                  System.out.println(d);
       

        	 
        }
     


        
        	
    }




   

} import java.io.*; io.* java import java.util.*; util.* java import java.math.*; math.* java class Main
{
    public static void main(String[] args)throws IOException
    {
        BufferedReader br =  new BufferedReader(new InputStreamReader(System.in));
        Scanner sc = new Scanner(System.in);
        int t = 1;
        while(t-->0)
        {
        	   int r = sc.nextInt();
                  double pie=3.141592653589793238;
                  double d = 2.0*pie*r;
                  System.out.println(d);
       

        	 
        }
     


        
        	
    }




   

} Main public static void main(String[] args)throws IOException
    {
        BufferedReader br =  new BufferedReader(new InputStreamReader(System.in));
        Scanner sc = new Scanner(System.in);
        int t = 1;
        while(t-->0)
        {
        	   int r = sc.nextInt();
                  double pie=3.141592653589793238;
                  double d = 2.0*pie*r;
                  System.out.println(d);
       

        	 
        }
     


        
        	
    } main {
        BufferedReader br =  new BufferedReader(new InputStreamReader(System.in));
        Scanner sc = new Scanner(System.in);
        int t = 1;
        while(t-->0)
        {
        	   int r = sc.nextInt();
                  double pie=3.141592653589793238;
                  double d = 2.0*pie*r;
                  System.out.println(d);
       

        	 
        }
     


        
        	
    } BufferedReader br =  new BufferedReader(new InputStreamReader(System.in)); br new BufferedReader(new InputStreamReader(System.in)) Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int t = 1; t 1 while(t-->0)
        {
        	   int r = sc.nextInt();
                  double pie=3.141592653589793238;
                  double d = 2.0*pie*r;
                  System.out.println(d);
       

        	 
        } t-->0 t-- t 0 {
        	   int r = sc.nextInt();
                  double pie=3.141592653589793238;
                  double d = 2.0*pie*r;
                  System.out.println(d);
       

        	 
        } int r = sc.nextInt(); r sc.nextInt() sc.nextInt sc double pie=3.141592653589793238; pie 3.141592653589793238 double d = 2.0*pie*r; d 2.0*pie*r 2.0*pie*r 2.0 pie r System.out.println(d) System.out.println System.out System System.out d String[] args args
import java.io.*;
import java.util.*;
import java.math.*;
 
class Main
{
    public static void main(String[] args)throws IOException
    {
        BufferedReader br =  new BufferedReader(new InputStreamReader(System.in));
        Scanner sc = new Scanner(System.in);
        int t = 1;
        while(t-->0)
        {
        	   int r = sc.nextInt();
                  double pie=3.141592653589793238;
                  double d = 2.0*pie*r;
                  System.out.println(d);
       

        	 
        }
     


        
        	
    }




   

}