import java.util.Scanner;

// import java.util.*;

public class Main {

    Scanner sc = new Scanner(System.in);
    public static void main(String[] args){
        new Main().run();
    }

    private void run(){
        int r = sc.nextInt();

        System.out.println(2*Math.PI*r);
    }
} import java.util.Scanner; Scanner java.util public class Main {

    Scanner sc = new Scanner(System.in);
    public static void main(String[] args){
        new Main().run();
    }

    private void run(){
        int r = sc.nextInt();

        System.out.println(2*Math.PI*r);
    }
} Main Scanner sc = new Scanner(System.in); sc new Scanner(System.in) public static void main(String[] args){
        new Main().run();
    } main {
        new Main().run();
    } new Main().run() new Main().run new Main() String[] args args private void run(){
        int r = sc.nextInt();

        System.out.println(2*Math.PI*r);
    } run {
        int r = sc.nextInt();

        System.out.println(2*Math.PI*r);
    } int r = sc.nextInt(); r sc.nextInt() sc.nextInt sc System.out.println(2*Math.PI*r) System.out.println System.out System System.out 2*Math.PI*r 2*Math.PI*r 2 Math.PI Math Math.PI r public class Main {

    Scanner sc = new Scanner(System.in);
    public static void main(String[] args){
        new Main().run();
    }

    private void run(){
        int r = sc.nextInt();

        System.out.println(2*Math.PI*r);
    }
} public class Main {

    Scanner sc = new Scanner(System.in);
    public static void main(String[] args){
        new Main().run();
    }

    private void run(){
        int r = sc.nextInt();

        System.out.println(2*Math.PI*r);
    }
} Main
import java.util.Scanner;

// import java.util.*;

public class Main {

    Scanner sc = new Scanner(System.in);
    public static void main(String[] args){
        new Main().run();
    }

    private void run(){
        int r = sc.nextInt();

        System.out.println(2*Math.PI*r);
    }
}
