import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		double pi = Math.PI;
		Scanner sc = new Scanner(System.in);
		int a =sc.nextInt();
		
		double ans= a*2*pi;
		
		System.out.print(ans);
	}

} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		double pi = Math.PI;
		Scanner sc = new Scanner(System.in);
		int a =sc.nextInt();
		
		double ans= a*2*pi;
		
		System.out.print(ans);
	}

} Main public static void main(String[] args) {
		// TODO Auto-generated method stub
		double pi = Math.PI;
		Scanner sc = new Scanner(System.in);
		int a =sc.nextInt();
		
		double ans= a*2*pi;
		
		System.out.print(ans);
	} main {
		// TODO Auto-generated method stub
		double pi = Math.PI;
		Scanner sc = new Scanner(System.in);
		int a =sc.nextInt();
		
		double ans= a*2*pi;
		
		System.out.print(ans);
	} double pi = Math.PI; pi Math.PI Math Math.PI Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a =sc.nextInt(); a sc.nextInt() sc.nextInt sc double ans= a*2*pi; ans a*2*pi a*2*pi a 2 pi System.out.print(ans) System.out.print System.out System System.out ans String[] args args public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		double pi = Math.PI;
		Scanner sc = new Scanner(System.in);
		int a =sc.nextInt();
		
		double ans= a*2*pi;
		
		System.out.print(ans);
	}

} public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		double pi = Math.PI;
		Scanner sc = new Scanner(System.in);
		int a =sc.nextInt();
		
		double ans= a*2*pi;
		
		System.out.print(ans);
	}

} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		double pi = Math.PI;
		Scanner sc = new Scanner(System.in);
		int a =sc.nextInt();
		
		double ans= a*2*pi;
		
		System.out.print(ans);
	}

}