import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
	// write your code here


        int R;
       


Scanner scan = new Scanner(System.in);
 R=scan.nextInt();



        double L;
        L=R*2*3.14;
        System.out.println(L);




    }
}

/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz