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

int main() {

  int a,l,x;
  while(cin >> a >> l >>x) {

    double h = l*l-(a/2.0)*(a/2.0);
    h = sqrt(h);
    double s = a * h / 2.0;

    h = ((l+x)/2.0)*((l+x)/2.0)-(l/2.0)*(l/2.0);
    h = sqrt(h);
    s += l*h;
    // cout << h << endl;
    //cout << s << endl;
    printf("%.10f\n",s);
  }

  return 0;
}