import java.util.Scanner;
import java.util.Set;
import java.util.TreeSet;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;

 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);
	  
	
	}

}