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

int main(){
  double a, l, x;

  while(cin >> a >> l >> x){
    double len = sqrt(l * l - (a / 2.0) * (a / 2.0));
    double b = (l + x) / 2.0;
    double s = (b + b + l) / 2.0;
    printf("%.10f\n", a * len / 2.0 + sqrt(s * (s - b) * (s - b) * (s - l)) * 2.0);
  }
}