import java.util.*;
import java.io.*;
public class Main
{
  public static void main(String args[])
  {
    Scanner sc=new Scanner(System.in);
    double pi=3.14159265;
    double rad;
    rad=sc.nextDouble();
    double cir;
    cir=2*pi*rad;
    System.out.printf("%.2f" ,cir);
  }
} 
/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