import java.util.Scanner;
public class Main{
  public static void main(String[] args){
    Scanner s = new Scanner(System.in);
    System.out.println(s.nextLong()*Math.PI*2);
  }
} import java.util.Scanner; Scanner java.util public class Main{
  public static void main(String[] args){
    Scanner s = new Scanner(System.in);
    System.out.println(s.nextLong()*Math.PI*2);
  }
} Main public static void main(String[] args){
    Scanner s = new Scanner(System.in);
    System.out.println(s.nextLong()*Math.PI*2);
  } main {
    Scanner s = new Scanner(System.in);
    System.out.println(s.nextLong()*Math.PI*2);
  } Scanner s = new Scanner(System.in); s new Scanner(System.in) System.out.println(s.nextLong()*Math.PI*2) System.out.println System.out System System.out s.nextLong()*Math.PI*2 s.nextLong()*Math.PI*2 s.nextLong() s.nextLong s Math.PI Math Math.PI 2 String[] args args public class Main{
  public static void main(String[] args){
    Scanner s = new Scanner(System.in);
    System.out.println(s.nextLong()*Math.PI*2);
  }
} public class Main{
  public static void main(String[] args){
    Scanner s = new Scanner(System.in);
    System.out.println(s.nextLong()*Math.PI*2);
  }
} Main
import java.util.Scanner;
public class Main{
  public static void main(String[] args){
    Scanner s = new Scanner(System.in);
    System.out.println(s.nextLong()*Math.PI*2);
  }
}