#include <stdio.h>
#include <math.h>
int main()
{
	double a, b, c;//三边长
	scanf("%lf %lf %lf", &a, &b, &c);
	double p = (1.0/2.0) * (a + b + c);
	double s = sqrt(p * (p - a) * (p - b) * (p - c));
	//四舍五入到一位
	s *= 10;
	s+= 0.5;
	s /= 10;
	printf("%.1lf", s);
	return 0;


}