/* package codechef; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;



/* Name of the class has to be "Main" only if the class is public. */
public class Main
 
 
{
    static Scanner in = new Scanner(System.in);
    
    public static void solve()
    {
        double r=in.nextDouble();
        
        System.out.println(2*Math.PI*r);
    }
    

    
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		
		int t=1;
		//in.nextLine();
		
		while(t>0)
		{
		    
		    solve();
		    t--;
		}
		
		
	
	//	System.out.println(x);
	}
} import java.util.*; util.* java import java.lang.*; lang.* java import java.io.*; io.* java public class Main
 
 
{
    static Scanner in = new Scanner(System.in);
    
    public static void solve()
    {
        double r=in.nextDouble();
        
        System.out.println(2*Math.PI*r);
    }
    

    
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		
		int t=1;
		//in.nextLine();
		
		while(t>0)
		{
		    
		    solve();
		    t--;
		}
		
		
	
	//	System.out.println(x);
	}
} Main static Scanner in = new Scanner(System.in); in new Scanner(System.in) public static void solve()
    {
        double r=in.nextDouble();
        
        System.out.println(2*Math.PI*r);
    } solve {
        double r=in.nextDouble();
        
        System.out.println(2*Math.PI*r);
    } double r=in.nextDouble(); r in.nextDouble() in.nextDouble in System.out.println(2*Math.PI*r) System.out.println System.out System System.out 2*Math.PI*r 2*Math.PI*r 2 Math.PI Math Math.PI r public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		
		int t=1;
		//in.nextLine();
		
		while(t>0)
		{
		    
		    solve();
		    t--;
		}
		
		
	
	//	System.out.println(x);
	} main {
		// your code goes here
		
		int t=1;
		//in.nextLine();
		
		while(t>0)
		{
		    
		    solve();
		    t--;
		}
		
		
	
	//	System.out.println(x);
	} int t=1; t 1 while(t>0)
		{
		    
		    solve();
		    t--;
		} t>0 t 0 {
		    
		    solve();
		    t--;
		} solve() solve t-- t String[] args args public class Main
 
 
{
    static Scanner in = new Scanner(System.in);
    
    public static void solve()
    {
        double r=in.nextDouble();
        
        System.out.println(2*Math.PI*r);
    }
    

    
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		
		int t=1;
		//in.nextLine();
		
		while(t>0)
		{
		    
		    solve();
		    t--;
		}
		
		
	
	//	System.out.println(x);
	}
} public class Main
 
 
{
    static Scanner in = new Scanner(System.in);
    
    public static void solve()
    {
        double r=in.nextDouble();
        
        System.out.println(2*Math.PI*r);
    }
    

    
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		
		int t=1;
		//in.nextLine();
		
		while(t>0)
		{
		    
		    solve();
		    t--;
		}
		
		
	
	//	System.out.println(x);
	}
} Main
/* package codechef; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;



/* Name of the class has to be "Main" only if the class is public. */
public class Main
 
 
{
    static Scanner in = new Scanner(System.in);
    
    public static void solve()
    {
        double r=in.nextDouble();
        
        System.out.println(2*Math.PI*r);
    }
    

    
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		
		int t=1;
		//in.nextLine();
		
		while(t>0)
		{
		    
		    solve();
		    t--;
		}
		
		
	
	//	System.out.println(x);
	}
}
