//solution 1
#include <bits/stdc++.h>
using namespace std;

using ll = long long;
#define fi first
#define se second

int main(){
  long double x,y,z,w;
  cin>>x>>y>>z>>w;
  cout<<fixed<<setprecision(10);
  long double minab=z,maxab=x/y;
  if(minab>maxab){
    swap(minab,maxab);
  }
  while(maxab-minab>1e-12){
    long double ab=(minab+maxab)/2;
    long double ad=x/ab;
    long double bp=sqrt(ab*ab+y*y),bq=sqrt(ad*ad+z*z);
    if(bp-bq>w){
      maxab=ab;
    }
    else{
      minab=ab;
    }
  }
  long double abp=(minab*y)/2,cbq=((x/minab)*z)/2,dpq=((minab-z)*(x/minab-y))/2;
  cout<<x-abp-cbq-dpq<<endl;
}