#include<bits/stdc++.h> using namespace std; int main() { float x,y,z,q,s; cin>>x>>y>>z; q=(x+y+z)/2; if((x+y>z)&&(x+z>y)&&(z+y>x))

 {
 	 s=sqrt(q*(q-x)*(q-z)*(q-y));
 printf("%.2lf",s);
   } 

else
cout<<"can't'";

return 0;

}