import java.util.AbstractList;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.PriorityQueue;
import java.util.Scanner;
import java.util.Iterator;
import java.util.Set;
import java.util.Stack;
import java.util.Vector;
import java.util.HashSet;

 public class ProblemA {
	 
	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
	    
		//inputs
		 
		int r = input.nextInt();
		
		System.out.println(2 * Math.PI * r);
	}
 }