#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define ALL(v) v.begin(), v.end()
typedef long long ll;

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

double EPS=1e-9;
double x,y,z,w;
double f(double a){
  return sqrt(a*a+y*y)-w-sqrt(x*x/a/a+z*z);
}

int main(){
  cin>>x>>y>>z>>w;
  
  double l=0,r=1e18;
  rep(i,50){
    double mid=(l+r)/2;
    if(f(mid)>EPS) r=mid;
    else if(f(mid)<EPS) l=mid;
    else break;
  }
  double a=r;
  double ans=x-a*y/2-x*z/2/a-(a-z)*(x/a-y)/2;
  
  cout<<fixed<<setprecision(10)<<ans<<endl;
  
  return 0;
}