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

struct tringle{
	double a;
	double b;
	double c;
}hen;

int main(void) {
	hen.a = 5;
	hen.b = 5;
	hen.c = 5;
	
	double q,r,s;
	q = (hen.a + hen.b + hen.c) / 2;
	r = q*(q - hen.a) * (q - hen.b) * (q - hen.c);
	s = sqrt(r);
	printf("%lf", s);
	
	return 0;
}