import java.util.*;
class  Main
{
    public static void main (String[] args) 
    {
       Scanner sc =new Scanner(System.in); 
       System.out.println();
       int r=sc.nextInt();
       double c=0.0d;
       c=2*3.14*r;
       System.out.println(c);
       
    }
}
/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