import java.util.Scanner;

class Main {
	// ABC164 A
	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		int R = Integer.parseInt(sc.next());

		double a = 2 * R * Math.PI;

		System.out.println(a);
	}

} import java.util.Scanner; Scanner java.util class Main {
	// ABC164 A
	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		int R = Integer.parseInt(sc.next());

		double a = 2 * R * Math.PI;

		System.out.println(a);
	}

} Main // ABC164 A
	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		int R = Integer.parseInt(sc.next());

		double a = 2 * R * Math.PI;

		System.out.println(a);
	} main {

		Scanner sc = new Scanner(System.in);

		int R = Integer.parseInt(sc.next());

		double a = 2 * R * Math.PI;

		System.out.println(a);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int R = Integer.parseInt(sc.next()); R Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc double a = 2 * R * Math.PI; a 2 * R * Math.PI 2 * R * Math.PI 2 R Math.PI Math Math.PI System.out.println(a) System.out.println System.out System System.out a String[] args args
import java.util.Scanner;

class Main {
	// ABC164 A
	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		int R = Integer.parseInt(sc.next());

		double a = 2 * R * Math.PI;

		System.out.println(a);
	}

}
