import java.util.Scanner;

public class Main {
	
	public static void main(String[] args)
	{
		Scanner s=new Scanner(System.in);
		
		double r=s.nextDouble();
		
		System.out.println(2*3.141592653589793238*r);
		
	}
	
} import java.util.Scanner; Scanner java.util public class Main {
	
	public static void main(String[] args)
	{
		Scanner s=new Scanner(System.in);
		
		double r=s.nextDouble();
		
		System.out.println(2*3.141592653589793238*r);
		
	}
	
} Main public static void main(String[] args)
	{
		Scanner s=new Scanner(System.in);
		
		double r=s.nextDouble();
		
		System.out.println(2*3.141592653589793238*r);
		
	} main {
		Scanner s=new Scanner(System.in);
		
		double r=s.nextDouble();
		
		System.out.println(2*3.141592653589793238*r);
		
	} Scanner s=new Scanner(System.in); s new Scanner(System.in) double r=s.nextDouble(); r s.nextDouble() s.nextDouble s System.out.println(2*3.141592653589793238*r) System.out.println System.out System System.out 2*3.141592653589793238*r 2*3.141592653589793238*r 2 3.141592653589793238 r String[] args args public class Main {
	
	public static void main(String[] args)
	{
		Scanner s=new Scanner(System.in);
		
		double r=s.nextDouble();
		
		System.out.println(2*3.141592653589793238*r);
		
	}
	
} public class Main {
	
	public static void main(String[] args)
	{
		Scanner s=new Scanner(System.in);
		
		double r=s.nextDouble();
		
		System.out.println(2*3.141592653589793238*r);
		
	}
	
} Main
import java.util.Scanner;

public class Main {
	
	public static void main(String[] args)
	{
		Scanner s=new Scanner(System.in);
		
		double r=s.nextDouble();
		
		System.out.println(2*3.141592653589793238*r);
		
	}
	
}
