import java.util.Scanner;

public class Main
{
    public  static  void main(String[] args)
    {
        Scanner sc = new Scanner(System.in);
        double a = sc.nextDouble();
        a = a*2*3.14159;
        System.out.print((a));

    }
}

/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