#include <iostream>
#include <cmath>
#include <cstdio>

using namespace std;

double calc(double a, double b){
  return sqrt(a * a - b * b);
}

int main(){
  double a, l, x, h, ans;
  while(cin >> a >> l >> x){
    h = calc(l, a / 2);
    ans = a * h / 2;
    h = calc((l + x) / 2, l / 2);
    ans += l * h;
    printf("%.10f\n", ans);
  }
}