import java.util.Scanner;

public class Main {
	public static void main(String[] args){
		Scanner stdIn = new Scanner(System.in);
		double R = stdIn.nextDouble();
		double ans = 0;

		ans = R * Math.PI * 2;
		System.out.println(ans);
		stdIn.close();
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args){
		Scanner stdIn = new Scanner(System.in);
		double R = stdIn.nextDouble();
		double ans = 0;

		ans = R * Math.PI * 2;
		System.out.println(ans);
		stdIn.close();
	}
} Main public static void main(String[] args){
		Scanner stdIn = new Scanner(System.in);
		double R = stdIn.nextDouble();
		double ans = 0;

		ans = R * Math.PI * 2;
		System.out.println(ans);
		stdIn.close();
	} main {
		Scanner stdIn = new Scanner(System.in);
		double R = stdIn.nextDouble();
		double ans = 0;

		ans = R * Math.PI * 2;
		System.out.println(ans);
		stdIn.close();
	} Scanner stdIn = new Scanner(System.in); stdIn new Scanner(System.in) double R = stdIn.nextDouble(); R stdIn.nextDouble() stdIn.nextDouble stdIn double ans = 0; ans 0 ans = R * Math.PI * 2 ans R * Math.PI * 2 R * Math.PI * 2 R Math.PI Math Math.PI 2 System.out.println(ans) System.out.println System.out System System.out ans stdIn.close() stdIn.close stdIn String[] args args public class Main {
	public static void main(String[] args){
		Scanner stdIn = new Scanner(System.in);
		double R = stdIn.nextDouble();
		double ans = 0;

		ans = R * Math.PI * 2;
		System.out.println(ans);
		stdIn.close();
	}
} public class Main {
	public static void main(String[] args){
		Scanner stdIn = new Scanner(System.in);
		double R = stdIn.nextDouble();
		double ans = 0;

		ans = R * Math.PI * 2;
		System.out.println(ans);
		stdIn.close();
	}
} Main
import java.util.Scanner;

public class Main {
	public static void main(String[] args){
		Scanner stdIn = new Scanner(System.in);
		double R = stdIn.nextDouble();
		double ans = 0;

		ans = R * Math.PI * 2;
		System.out.println(ans);
		stdIn.close();
	}
}