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

	}

}
/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