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