#include<iostream>
#include<cmath>

using namespace std;

int main(){
  double a, l, x, t1, t2, s;

  while(cin >> a >> l >> x){
    x += l;
    x /= 2;
    t1 = acos((2*l*l - a*a)/(2*l*l));
    t2 = acos((2*x*x - l*l)/(2*x*x));
    s = l*l*sin(t1)/2 + x*x*sin(t2);
    cout.setf(ios::fixed, ios::floatfield);
    cout.precision(10);
    cout << s << endl;
  }
  return 0;
}