import java.util.Scanner;

public class Main {

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

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

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

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

public class Main {

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