//package beg_163;
import java.util.*;
public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner s = new Scanner(System.in);
		
		int a = s.nextInt();
		
		double b= (double) a;
		
		System.out.println(2*b*3.14159265);

	}

} import java.util.*; util.* java public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner s = new Scanner(System.in);
		
		int a = s.nextInt();
		
		double b= (double) a;
		
		System.out.println(2*b*3.14159265);

	}

} Main public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner s = new Scanner(System.in);
		
		int a = s.nextInt();
		
		double b= (double) a;
		
		System.out.println(2*b*3.14159265);

	} main {
		// TODO Auto-generated method stub
		Scanner s = new Scanner(System.in);
		
		int a = s.nextInt();
		
		double b= (double) a;
		
		System.out.println(2*b*3.14159265);

	} Scanner s = new Scanner(System.in); s new Scanner(System.in) int a = s.nextInt(); a s.nextInt() s.nextInt s double b= (double) a; b (double) a System.out.println(2*b*3.14159265) System.out.println System.out System System.out 2*b*3.14159265 2*b*3.14159265 2 b 3.14159265 String[] args args public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner s = new Scanner(System.in);
		
		int a = s.nextInt();
		
		double b= (double) a;
		
		System.out.println(2*b*3.14159265);

	}

} public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner s = new Scanner(System.in);
		
		int a = s.nextInt();
		
		double b= (double) a;
		
		System.out.println(2*b*3.14159265);

	}

} Main
//package beg_163;
import java.util.*;
public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner s = new Scanner(System.in);
		
		int a = s.nextInt();
		
		double b= (double) a;
		
		System.out.println(2*b*3.14159265);

	}

}
