#include<stdio.h>
#include <math.h>
int main()
{
	float a, b, c, p, s;
	scanf("%f %f %f", &a, &b, &c);
	p = (a + b + c)/2;
	s = sqrt( p * (p - a) * (p - b) * (p - c) );
	printf("%.1f", s);
	return 0;
}