#include <bits/stdc++.h>
using namespace std;
#define EPS 0.00001

double calcS(double bottom, double l){
  return ((double)bottom * sqrt(pow(l, 2) - pow((double)bottom/2, 2))) / 2;
}

int main(void){
  int a, l, x;
  while(cin >> a >> l >> x){
    double dflt = calcS(a, l);
    double added = calcS(l, ((double)l+x)/2);
    printf("%.7f\n", dflt + added * 2 );
  }
  return 0;
}