import java.util.*;

public class Main {
public static void main(String args[]) {
	Scanner sc=new Scanner(System.in);
	double h=sc.nextDouble();
	double r=h*2*3.14;
	
    System.out.println(r);
}
}

/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