import java .util.*;

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

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