import java.util.Scanner;

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

		Scanner sc = new Scanner(System.in);

		int r = sc.nextInt();

		double X = r * 2 * Math.PI;

		System.out.print(X);

	return;

	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args){

		Scanner sc = new Scanner(System.in);

		int r = sc.nextInt();

		double X = r * 2 * Math.PI;

		System.out.print(X);

	return;

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

		Scanner sc = new Scanner(System.in);

		int r = sc.nextInt();

		double X = r * 2 * Math.PI;

		System.out.print(X);

	return;

	} main {

		Scanner sc = new Scanner(System.in);

		int r = sc.nextInt();

		double X = r * 2 * Math.PI;

		System.out.print(X);

	return;

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

		Scanner sc = new Scanner(System.in);

		int r = sc.nextInt();

		double X = r * 2 * Math.PI;

		System.out.print(X);

	return;

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

		Scanner sc = new Scanner(System.in);

		int r = sc.nextInt();

		double X = r * 2 * Math.PI;

		System.out.print(X);

	return;

	}
} Main
import java.util.Scanner;

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

		Scanner sc = new Scanner(System.in);

		int r = sc.nextInt();

		double X = r * 2 * Math.PI;

		System.out.print(X);

	return;

	}
}
