
#include<iostream>
#include<cmath>
using namespace std;

int main()
{
  double a,l,x;
  double s;
  cout<<setprecision(10)<<setiosflags(ios::fixed);
  while(cin>>a>>l>>x){
    x+=l;
    s = sqrt(l*l - a*a/4.0) * a / 2.0;
    s += sqrt(x*x/4.0 - l*l/4.0) * l;
    cout<<s<<endl;
  }
}