#include<stdio.h>
double root(double x)
{
	double min, mid, max;
	min = 0;
	max = 1;
	while (max * max < x)
		max *= 2;
	for (int i = 0; i < 100; i++)
	{
		mid = (max + min) / 2;
		if (mid * mid < x)
			min = mid;
		else
			max = mid;
	}
	return max;
}
int main()
{
	double l;
	scanf("%lf", &l);
	printf("%.20lf\n", root(3) * l * l / 36);
	return 0;
}