import java.util.AbstractList;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.PriorityQueue;
import java.util.Scanner;
import java.util.Iterator;
import java.util.Set;
import java.util.Stack;
import java.util.Vector;
import java.util.HashSet;

 public class  Main {
	 
	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
	    
		//inputs
		 
		int r = input.nextInt();
		
		System.out.println(2 * Math.PI * r);
	}
 } import java.util.AbstractList; AbstractList java.util import java.util.ArrayList; ArrayList java.util import java.util.Arrays; Arrays java.util import java.util.Collections; Collections java.util import java.util.Comparator; Comparator java.util import java.util.List; List java.util import java.util.PriorityQueue; PriorityQueue java.util import java.util.Scanner; Scanner java.util import java.util.Iterator; Iterator java.util import java.util.Set; Set java.util import java.util.Stack; Stack java.util import java.util.Vector; Vector java.util import java.util.HashSet; HashSet java.util public class  Main {
	 
	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
	    
		//inputs
		 
		int r = input.nextInt();
		
		System.out.println(2 * Math.PI * r);
	}
 } Main public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
	    
		//inputs
		 
		int r = input.nextInt();
		
		System.out.println(2 * Math.PI * r);
	} main {
		Scanner input = new Scanner(System.in);
	    
		//inputs
		 
		int r = input.nextInt();
		
		System.out.println(2 * Math.PI * r);
	} Scanner input = new Scanner(System.in); input new Scanner(System.in) int r = input.nextInt(); r input.nextInt() input.nextInt input System.out.println(2 * Math.PI * r) System.out.println System.out System System.out 2 * Math.PI * r 2 * Math.PI * r 2 Math.PI Math Math.PI r String[] args args public class  Main {
	 
	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
	    
		//inputs
		 
		int r = input.nextInt();
		
		System.out.println(2 * Math.PI * r);
	}
 } public class  Main {
	 
	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
	    
		//inputs
		 
		int r = input.nextInt();
		
		System.out.println(2 * Math.PI * r);
	}
 } Main
import java.util.AbstractList;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.PriorityQueue;
import java.util.Scanner;
import java.util.Iterator;
import java.util.Set;
import java.util.Stack;
import java.util.Vector;
import java.util.HashSet;

 public class  Main {
	 
	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
	    
		//inputs
		 
		int r = input.nextInt();
		
		System.out.println(2 * Math.PI * r);
	}
 }