/* -*- coding: utf-8 -*-
 *
 * 1381.cc:  No.1381 Simple Geometry 1 - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int CNT = 1000;

/* typedef */

/* global variables */

/* subroutines */

/* main */

int main() {
  double s, ap, cq, w;
  scanf("%lf%lf%lf%lf", &s, &ap, &cq, &w);

  // let x = |ad|, y = |cd|,
  // -> s = x*y, |bp|^2=|ap|^2+y^2, |bq|^2=x^2+|cq|^2
  //    x >= |ap|, y >= |cq| -> s=x*y>=x*|cq| -> x<=s/|cq|

  double ap2 = ap * ap, cq2 = cq * cq;

  double x0 = ap, x1 = s / cq;
  for (int cnt = 0; cnt < CNT; cnt++) {
    double x = (x0 + x1) / 2;
    double y = s / x;
    double bp = sqrt(ap2 + y * y);
    double bq = sqrt(x * x + cq2);

    if (bp >= bq + w) x0 = x;
    else x1 = x;
  }
  //printf("x=%lf, y=%lf\n", x0, s / x0);

  double y0 = s / x0;
  double t = s - (ap * y0 / 2 + x0 * cq / 2 + (x0 - ap) * (y0 - cq) / 2);
  printf("%.10lf\n", t);
  return 0;
}