include

include

include

using namespace std; int main(void){

//freopen("1118.in","r",stdin);
//freopen("1118.out","w",stdout);
double a,b,c,d,e;
scanf("%lf%lf%lf",&a,&b,&c);
if(a<b+c&&b<a+c&&c<a+b){
	d=(a+b+c)/2;
	e=sqrt(d*(d-a)*(d-b)*(d-c));
	printf("%.2lf",e);	
}else{
	printf("no");
}
return 0;

}