#include<stdio.h>
#include<math.h>

double main()
 {
	double a, b, c, p,s;
	scanf("%f%f%f", &a, &b, &c);
	p = (a + b + c) *0.5;
	s = sqrt(p * (p - a) * (p - b) * (p - c));
	printf("%.1lf", s);
	
}