import java.util.Scanner;
public class Main
{
    public static void main(String Args[])
    {
        Scanner sc=new Scanner (System.in);
        int n=sc.nextInt();
        double c=2*Math.PI*n;
        System.out.println(c);
    }
}
/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