import java.lang.Math;
import java.util.*;
 
class Main{
	public static void main(String args[]){
    	Scanner sc = new Scanner(System.in);
      	double i = sc.nextInt();
      	System.out.println(((double)Math.round(i*2*Math.PI*100))/100);
    
    
    }
 
 
} import java.lang.Math; Math java.lang import java.util.*; util.* java class Main{
	public static void main(String args[]){
    	Scanner sc = new Scanner(System.in);
      	double i = sc.nextInt();
      	System.out.println(((double)Math.round(i*2*Math.PI*100))/100);
    
    
    }
 
 
} Main public static void main(String args[]){
    	Scanner sc = new Scanner(System.in);
      	double i = sc.nextInt();
      	System.out.println(((double)Math.round(i*2*Math.PI*100))/100);
    
    
    } main {
    	Scanner sc = new Scanner(System.in);
      	double i = sc.nextInt();
      	System.out.println(((double)Math.round(i*2*Math.PI*100))/100);
    
    
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) double i = sc.nextInt(); i sc.nextInt() sc.nextInt sc System.out.println(((double)Math.round(i*2*Math.PI*100))/100) System.out.println System.out System System.out ((double)Math.round(i*2*Math.PI*100))/100 ((double)Math.round(i*2*Math.PI*100)) Math.round Math i*2*Math.PI*100 Math.PI i*2*Math.PI*100 i 2 Math.PI Math Math.PI 100 100 String args[] args
import java.lang.Math;
import java.util.*;
 
class Main{
	public static void main(String args[]){
    	Scanner sc = new Scanner(System.in);
      	double i = sc.nextInt();
      	System.out.println(((double)Math.round(i*2*Math.PI*100))/100);
    
    
    }
 
 
}