import java.util.Scanner;

public class Main {

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

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

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

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

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

} public class Main {

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

} Main
import java.util.Scanner;

public class Main {

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

}
