include<stdio.h>

include<math.h>

int main() {

double a,b,c,d,s;
scanf("%lf %lf %lf",&a,&b,&c);
if(a+b>c && a-b<c)
{
	d=(a+b+c)/2;
	s=sqrt(d*(d-a)*(d-b)*(d-c));
	printf("%.2f",s);
}
else
{
	printf("no");
}
return 0;

}