using namespace std;
double f(double a,double b,double c)
{
	double p=(a+b+c)/2.0;
	p=sqrt(p*(p-a)*(p-b)*(p-c));
	return p;
}
int main()
{
	double a,b,c;
	cin>>a>>b>>c;
	cout<<fixed<<setprecision(2)<<f(a,b,c)<<endl;
    return 0;
}