//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);
      
	}
 
}