import java.util.Arrays;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		new Main().solver();
	}

	void solver() {
		Scanner sc = new Scanner(System.in);
		int L = sc.nextInt();
		double ans = Math.sqrt(3) / 36 * L * L;
		System.out.println(ans);
	}
}