import java.util.Scanner; 

class Main{

 public static void main(String[] args){

 Scanner scan = new Scanner(System.in);

 int R = scan.nextInt();
 double a= 6.28318530717958623200;
 double number=a*R;

System.out.println(number);

 }
}
/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