import java.util.Scanner;

public class Main {
	public static void main(String[] args){
		try(Scanner sc = new Scanner(System.in)) {
			

			double r = sc.nextDouble();
			
			System.out.println(Math.PI * 2f * r);
		}
	}

} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args){
		try(Scanner sc = new Scanner(System.in)) {
			

			double r = sc.nextDouble();
			
			System.out.println(Math.PI * 2f * r);
		}
	}

} Main public static void main(String[] args){
		try(Scanner sc = new Scanner(System.in)) {
			

			double r = sc.nextDouble();
			
			System.out.println(Math.PI * 2f * r);
		}
	} main {
		try(Scanner sc = new Scanner(System.in)) {
			

			double r = sc.nextDouble();
			
			System.out.println(Math.PI * 2f * r);
		}
	} try(Scanner sc = new Scanner(System.in)) {
			

			double r = sc.nextDouble();
			
			System.out.println(Math.PI * 2f * r);
		} {
			

			double r = sc.nextDouble();
			
			System.out.println(Math.PI * 2f * r);
		} Scanner sc = new Scanner(System.in) Scanner sc = new Scanner(System.in) new Scanner(System.in) double r = sc.nextDouble(); r sc.nextDouble() sc.nextDouble sc System.out.println(Math.PI * 2f * r) System.out.println System.out System System.out Math.PI * 2f * r Math.PI * 2f * r Math.PI Math Math.PI 2f r String[] args args public class Main {
	public static void main(String[] args){
		try(Scanner sc = new Scanner(System.in)) {
			

			double r = sc.nextDouble();
			
			System.out.println(Math.PI * 2f * r);
		}
	}

} public class Main {
	public static void main(String[] args){
		try(Scanner sc = new Scanner(System.in)) {
			

			double r = sc.nextDouble();
			
			System.out.println(Math.PI * 2f * r);
		}
	}

} Main
import java.util.Scanner;

public class Main {
	public static void main(String[] args){
		try(Scanner sc = new Scanner(System.in)) {
			

			double r = sc.nextDouble();
			
			System.out.println(Math.PI * 2f * r);
		}
	}

}