import java.util.Scanner;
public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		double r = sc.nextDouble();
		r = 2.0 * r * Math.PI;
		System.out.println(r);
		
	}
} import java.util.Scanner; Scanner java.util public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		double r = sc.nextDouble();
		r = 2.0 * r * Math.PI;
		System.out.println(r);
		
	}
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		double r = sc.nextDouble();
		r = 2.0 * r * Math.PI;
		System.out.println(r);
		
	} main {
		Scanner sc = new Scanner(System.in);
		double r = sc.nextDouble();
		r = 2.0 * r * Math.PI;
		System.out.println(r);
		
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) double r = sc.nextDouble(); r sc.nextDouble() sc.nextDouble sc r = 2.0 * r * Math.PI r 2.0 * r * Math.PI 2.0 * r * Math.PI 2.0 r Math.PI Math Math.PI System.out.println(r) System.out.println System.out System System.out r String[] args args public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		double r = sc.nextDouble();
		r = 2.0 * r * Math.PI;
		System.out.println(r);
		
	}
} public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		double r = sc.nextDouble();
		r = 2.0 * r * Math.PI;
		System.out.println(r);
		
	}
} Main
import java.util.Scanner;
public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		double r = sc.nextDouble();
		r = 2.0 * r * Math.PI;
		System.out.println(r);
		
	}
}