#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;
}