import java.lang.Math;
import java.util.*;
 
class Main{
	public static void main(String args[]){
    	Scanner sc = new Scanner(System.in);
      	double i = sc.nextInt();
      	System.out.println(((double)Math.round(i*2*Math.PI*100))/100);
    
    
    }
 
 
}
/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