import java.util.Scanner;

public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int R = sc.nextInt();
		sc.close();

		double ret = getProcess(R);

		System.out.println(ret);
	}

	public static double getProcess(int R){
		double ret = 2 * Math.PI * R ;

		return ret;
	}


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

		double ret = getProcess(R);

		System.out.println(ret);
	}

	public static double getProcess(int R){
		double ret = 2 * Math.PI * R ;

		return ret;
	}


} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int R = sc.nextInt();
		sc.close();

		double ret = getProcess(R);

		System.out.println(ret);
	} main {
		Scanner sc = new Scanner(System.in);
		int R = sc.nextInt();
		sc.close();

		double ret = getProcess(R);

		System.out.println(ret);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int R = sc.nextInt(); R sc.nextInt() sc.nextInt sc sc.close() sc.close sc double ret = getProcess(R); ret getProcess(R) getProcess R System.out.println(ret) System.out.println System.out System System.out ret String[] args args public static double getProcess(int R){
		double ret = 2 * Math.PI * R ;

		return ret;
	} getProcess {
		double ret = 2 * Math.PI * R ;

		return ret;
	} double ret = 2 * Math.PI * R ; ret 2 * Math.PI * R 2 * Math.PI * R 2 Math.PI Math Math.PI R return ret; ret int R R public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int R = sc.nextInt();
		sc.close();

		double ret = getProcess(R);

		System.out.println(ret);
	}

	public static double getProcess(int R){
		double ret = 2 * Math.PI * R ;

		return ret;
	}


} public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int R = sc.nextInt();
		sc.close();

		double ret = getProcess(R);

		System.out.println(ret);
	}

	public static double getProcess(int R){
		double ret = 2 * Math.PI * R ;

		return ret;
	}


} Main
import java.util.Scanner;

public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int R = sc.nextInt();
		sc.close();

		double ret = getProcess(R);

		System.out.println(ret);
	}

	public static double getProcess(int R){
		double ret = 2 * Math.PI * R ;

		return ret;
	}


}