//keshavs first usaco solve :)
 
import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintWriter;
import java.util.Scanner;
 
public class Main {
 
	public static void main(String[] args) throws FileNotFoundException {
		// TODO Auto-generated method stub
 
		Scanner sc = new Scanner(System.in);
		double r = sc.nextInt();
      	System.out.println(r*Math.PI*2);
      
	}
 
} import java.io.File; File java.io import java.io.FileNotFoundException; FileNotFoundException java.io import java.io.PrintWriter; PrintWriter java.io import java.util.Scanner; Scanner java.util public class Main {
 
	public static void main(String[] args) throws FileNotFoundException {
		// TODO Auto-generated method stub
 
		Scanner sc = new Scanner(System.in);
		double r = sc.nextInt();
      	System.out.println(r*Math.PI*2);
      
	}
 
} Main public static void main(String[] args) throws FileNotFoundException {
		// TODO Auto-generated method stub
 
		Scanner sc = new Scanner(System.in);
		double r = sc.nextInt();
      	System.out.println(r*Math.PI*2);
      
	} main {
		// TODO Auto-generated method stub
 
		Scanner sc = new Scanner(System.in);
		double r = sc.nextInt();
      	System.out.println(r*Math.PI*2);
      
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) double r = sc.nextInt(); r sc.nextInt() sc.nextInt sc System.out.println(r*Math.PI*2) System.out.println System.out System System.out r*Math.PI*2 r*Math.PI*2 r Math.PI Math Math.PI 2 String[] args args public class Main {
 
	public static void main(String[] args) throws FileNotFoundException {
		// TODO Auto-generated method stub
 
		Scanner sc = new Scanner(System.in);
		double r = sc.nextInt();
      	System.out.println(r*Math.PI*2);
      
	}
 
} public class Main {
 
	public static void main(String[] args) throws FileNotFoundException {
		// TODO Auto-generated method stub
 
		Scanner sc = new Scanner(System.in);
		double r = sc.nextInt();
      	System.out.println(r*Math.PI*2);
      
	}
 
} Main
//keshavs first usaco solve :)
 
import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintWriter;
import java.util.Scanner;
 
public class Main {
 
	public static void main(String[] args) throws FileNotFoundException {
		// TODO Auto-generated method stub
 
		Scanner sc = new Scanner(System.in);
		double r = sc.nextInt();
      	System.out.println(r*Math.PI*2);
      
	}
 
}