#include <stdio.h>
#include <math.h>
int main(void) {
	// your code goes here
double a,b;
double c,d,e;
scanf("%lf %lf %lf",&c,&d,&e);
b=(c+d+e)/2;
a=sqrt(b*(b-c)*(b-d)*(b-e));
printf("s=%.2f",a);
	return 0;
}