import java.util.Scanner;
public class Main{
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		System.out.println(answer(scan.nextInt()));
	}
	public static double answer(int radius){
		return (double)(2 * Math.PI * radius);
	}
} import java.util.Scanner; Scanner java.util public class Main{
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		System.out.println(answer(scan.nextInt()));
	}
	public static double answer(int radius){
		return (double)(2 * Math.PI * radius);
	}
} Main public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		System.out.println(answer(scan.nextInt()));
	} main {
		Scanner scan = new Scanner(System.in);
		System.out.println(answer(scan.nextInt()));
	} Scanner scan = new Scanner(System.in); scan new Scanner(System.in) System.out.println(answer(scan.nextInt())) System.out.println System.out System System.out answer(scan.nextInt()) answer scan.nextInt() scan.nextInt scan String[] args args public static double answer(int radius){
		return (double)(2 * Math.PI * radius);
	} answer {
		return (double)(2 * Math.PI * radius);
	} return (double)(2 * Math.PI * radius); (double)(2 * Math.PI * radius) 2 * Math.PI * radius 2 Math.PI Math Math.PI radius int radius radius public class Main{
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		System.out.println(answer(scan.nextInt()));
	}
	public static double answer(int radius){
		return (double)(2 * Math.PI * radius);
	}
} public class Main{
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		System.out.println(answer(scan.nextInt()));
	}
	public static double answer(int radius){
		return (double)(2 * Math.PI * radius);
	}
} Main
import java.util.Scanner;
public class Main{
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		System.out.println(answer(scan.nextInt()));
	}
	public static double answer(int radius){
		return (double)(2 * Math.PI * radius);
	}
}