import java.util.Scanner;

class Main{
 public static void main(String[] args){
  Scanner stdIn = new Scanner(System.in);
   final double pi = 3.141;
   int R = stdIn.nextInt();
   double Enshu = 2 * pi * R;
   
   System.out.println(Enshu);
 }
}
/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