//keshavs first usaco solve :)

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(new File("mixmilk.in"));
		int r = sc.nextInt();
      	System.our.println(r*Math.PI*2);
      
	}

}
