import java.util.Scanner;
public class Circumference {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
Scanner sc = new Scanner (System.in);
float pi=0;
int R=0;
 R = sc.nextInt();
float Circumference = sc.nextFloat();
Circumference = 2 * pi * R;
System.out.println(Circumference  );
	}


	}


