import java.lang.Math;
import java.util.Scanner;

public class Main
{
        public static void main( String[] args )
        {
                Scanner scan = new Scanner(System.in);
                String str = scan.nextLine();

                System.out.println(Integer.parseInt(str) * 2 * Math.PI);
                scan.close();

        }
}

/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