#include<stdio.h>
double sqrt(double x)
{
	double min, mid, max;
	min = 0;
	max = 1;
	while (max * max <= x)
		max *= 2;
	int i;
	for (i = 0; i < 100; i++)
	{
		mid = (max + min) / 2;
		if (mid * mid > x)
			max = mid;
		else
			min = mid;
	}
	return min;
}
int main()
{
	double a, b, c;
	scanf("%lf %lf %lf", &a, &b, &c);
	double s = (a + b + c) / 2;
	double S = sqrt(s * (s - a) * (s - b) * (s - c));
	printf("%.20lf\n", S / 4);
	return 0;
}