import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		double R = scan.nextInt();

		System.out.println(2*R*Math.PI);

	}

} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		double R = scan.nextInt();

		System.out.println(2*R*Math.PI);

	}

} Main public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		double R = scan.nextInt();

		System.out.println(2*R*Math.PI);

	} main {

		Scanner scan = new Scanner(System.in);

		double R = scan.nextInt();

		System.out.println(2*R*Math.PI);

	} Scanner scan = new Scanner(System.in); scan new Scanner(System.in) double R = scan.nextInt(); R scan.nextInt() scan.nextInt scan System.out.println(2*R*Math.PI) System.out.println 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 scan = new Scanner(System.in);

		double R = scan.nextInt();

		System.out.println(2*R*Math.PI);

	}

} public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		double R = scan.nextInt();

		System.out.println(2*R*Math.PI);

	}

} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		double R = scan.nextInt();

		System.out.println(2*R*Math.PI);

	}

}