import java.util.Scanner;
public class Main{
public static void main(String[]args){

Scanner sc=new Scanner(System.in);

int R =sc.nextInt();
double ans=R*6.28318530717958623200;
  System.out.println(ans);

}
}
/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