#include
#include
double p(int a,int b,int c)
{
double s,r;
if(a+b>c&&a+c>b&&b+c>a)
{
r=(a+b+c)/2;//<====================
s=sqrt(r*(r-a)*(r-b)*(r-c));
return s;
} else return 0;
}
main()
{
int x,y,z;
double q;
{
}
while(1)
{
scanf("%d %d %d",&x,&y,&z);
if(x!=0&&y!=0&&z!=0) break;//<==============
}
{
q=p(x,y,z);
if ( p(x,y,z)==0)printf("NO\n"); else printf("%.2lf\n",q);
}
}