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