#include<cstdio>

 #include<cmath>

 #include<algorithm>

 #include<iostream>

 #include<cstring>

 using namespace std;

 typedef long long ll;

 const double eps = 1e-;

 int sgn(double x)

 {

     if(fabs(x) < eps) return ;

     else return x <  ? - : ;

 }

 int main()

 {

     double a, b ,c;

     while(~scanf("%lf %lf %lf",&a,&b,&c))

     {

         if(a!= && b !=  && c !=  && sgn(a + b - c) > && sgn( a + c - b ) >  &&sgn( b + c - a) > )

         {

             double p =a + b + c;

             double Elem1 = p -*a;

             double Elem2 = p -*b;

             double Elem3 = p -*c;

             double S=sqrt(p *Elem1*Elem2*Elem3);

             S/=;

             printf("%.3f\n",S);

         }

         else

            printf("-1.000\n");

     }

     return ;

 }