import java.util.Scanner;

public class Main{
public static void main(String[]args){
  final double PI = 3.141592;
  Scanner stdIn = new Scanner(System.in);
  double r = stdIn.nextDouble();
  System.out.println(2 * PI * r );

 }
 }
/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