#include<iostream>
#include<iomanip>
#include<complex>
using namespace std;
#define EPS 1e-10
double area2( double a, double b, double c){
  double s = ( a + b + c ) / 2;
  return sqrt(s * (s - a) * (s - b) * (s - c));
}

int main(){
  int a, l, x;
  while(cin >> a >> l >> x){
    double h = (l + x) / 2.0; // たぶん真ん中でしょ
    cout << fixed << setprecision(6) << area2(a,l,l) + area2(h,h,l)*2 << endl;
  }
}