import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int l = sc.nextInt();
		sc.close();

		double l2 = l / 2.0;
		double l3 = l / 3.0;
		System.out.println(Math.sqrt(l2 * Math.pow(l2 - l3, 3)));
	}
}