import java.io.BufferedReader; 
import java.io.IOException; 
import java.io.InputStreamReader; 
import java.util.StringTokenizer; 
import java.util.*;
 
public class Solution 
{ 
  
    
    public static void main(String[] args) throws IOException 
    { 
  
       BufferedReader br = new BufferedReader( 
                              new InputStreamReader(System.in));
                               int n =  Integer.parseInt(br.readLine());
                 
                   System.out.println(6.28318530717958623200*n);
                
                     
    }
    }
 
