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);
       

        	 
        }
     


        
        	
    }




   

}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz