import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		final double π = 3.14159265359;

		Scanner scan = new Scanner(System.in);
		int r = Integer.parseInt(scan.next());
		double pre = 2 * π;
		double res = pre * r;

		System.out.println(res);
	}
} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {

		final double π = 3.14159265359;

		Scanner scan = new Scanner(System.in);
		int r = Integer.parseInt(scan.next());
		double pre = 2 * π;
		double res = pre * r;

		System.out.println(res);
	}
} Main public static void main(String[] args) {

		final double π = 3.14159265359;

		Scanner scan = new Scanner(System.in);
		int r = Integer.parseInt(scan.next());
		double pre = 2 * π;
		double res = pre * r;

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

		final double π = 3.14159265359;

		Scanner scan = new Scanner(System.in);
		int r = Integer.parseInt(scan.next());
		double pre = 2 * π;
		double res = pre * r;

		System.out.println(res);
	} final double π = 3.14159265359; π 3.14159265359 Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int r = Integer.parseInt(scan.next()); r Integer.parseInt(scan.next()) Integer.parseInt Integer scan.next() scan.next scan double pre = 2 * π; pre 2 * π 2 π double res = pre * r; res pre * r pre r System.out.println(res) System.out.println System.out System System.out res String[] args args public class Main {

	public static void main(String[] args) {

		final double π = 3.14159265359;

		Scanner scan = new Scanner(System.in);
		int r = Integer.parseInt(scan.next());
		double pre = 2 * π;
		double res = pre * r;

		System.out.println(res);
	}
} public class Main {

	public static void main(String[] args) {

		final double π = 3.14159265359;

		Scanner scan = new Scanner(System.in);
		int r = Integer.parseInt(scan.next());
		double pre = 2 * π;
		double res = pre * r;

		System.out.println(res);
	}
} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		final double π = 3.14159265359;

		Scanner scan = new Scanner(System.in);
		int r = Integer.parseInt(scan.next());
		double pre = 2 * π;
		double res = pre * r;

		System.out.println(res);
	}
}
