#include <iostream>
#include <cmath>
using namespace std;
int main()
{double triangle(double,double,double);
 double a,b,c;
 cin>>a>>b>>c;
 try
  {while(a>0 && b>0 && c>0)
    {cout<<triangle(a,b,c)<<endl;
     cin>>a>>b>>c;}
  }
 catch(double)
  {cout<<"a="<<a<<",b="<<b<<",c="<<c<<",that is not a traingle!"<<endl;}
 cout<<"end"<<endl;
 return 0;
}
  
double triangle(double a,double b,double c)
{double s=(a+b+c)/2;
 if (a+b<=c||b+c<=a||c+a<=b) throw a;
 return sqrt(s*(s-a)*(s-b)*(s-c));
}