

import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintWriter;
import java.util.Scanner;

public class Solution {

	public static void main(String[] args) throws FileNotFoundException {
		// TODO Auto-generated method stub

		Scanner sc = new Scanner(System.in);
      	int radius = sc.nextInt();
      
      	System.out.println(2*Math.PI*radius);
		

}
