import java.util.*;
public class Main{
  public static void main(String[]args){
  Scanner sc=new Scanner(System.in);
  int n=sc.nextInt();
  double c=2*3.141592653589793238*n;
  System.out.println(c);
  
  }
} import java.util.*; util.* java public class Main{
  public static void main(String[]args){
  Scanner sc=new Scanner(System.in);
  int n=sc.nextInt();
  double c=2*3.141592653589793238*n;
  System.out.println(c);
  
  }
} Main public static void main(String[]args){
  Scanner sc=new Scanner(System.in);
  int n=sc.nextInt();
  double c=2*3.141592653589793238*n;
  System.out.println(c);
  
  } main {
  Scanner sc=new Scanner(System.in);
  int n=sc.nextInt();
  double c=2*3.141592653589793238*n;
  System.out.println(c);
  
  } Scanner sc=new Scanner(System.in); sc new Scanner(System.in) int n=sc.nextInt(); n sc.nextInt() sc.nextInt sc double c=2*3.141592653589793238*n; c 2*3.141592653589793238*n 2*3.141592653589793238*n 2 3.141592653589793238 n System.out.println(c) System.out.println System.out System System.out c String[]args args public class Main{
  public static void main(String[]args){
  Scanner sc=new Scanner(System.in);
  int n=sc.nextInt();
  double c=2*3.141592653589793238*n;
  System.out.println(c);
  
  }
} public class Main{
  public static void main(String[]args){
  Scanner sc=new Scanner(System.in);
  int n=sc.nextInt();
  double c=2*3.141592653589793238*n;
  System.out.println(c);
  
  }
} Main
import java.util.*;
public class Main{
  public static void main(String[]args){
  Scanner sc=new Scanner(System.in);
  int n=sc.nextInt();
  double c=2*3.141592653589793238*n;
  System.out.println(c);
  
  }
}