import java.util.*;
public class Main{

	public static void main(String[]args){
    	Scanner sc=new Scanner(System.in);
      	int t=sc.nextInt();
      	System.out.println(t*6.28318530717958623200);
    
    
    }

}

/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