#include <bits/stdc++.h>
using namespace std;
#define ll long long
const int N = 2e4 + 9;
const int mod = 1e9 + 7;
const double eps = 1e-9; 
#define inf 2e18

double h,r,da,dw;
double ans;
struct Node
{    
    double pos,val;
}a[N];

double cal(double x)
{
    double res= (h * h * da + x * x *(dw - da)) / (h * da + x * (dw - da));
    return res;
}


void solve()
{
    cin>>h>>r>>da>>dw;
    //ans = (h * h * da + h * h *(dw - da)) / (h * da + h * (dw - da));
    double lpos=1,rpos=1000;
    int pos;
    double now=1,d=(rpos - lpos)/1000;
    a[0]={0,1e16},a[1001]={1001,1e16};
    for(int i=1;i<=1000;++i)  
    {
        a[i]={now,cal(now)};
        now += d;
    }

    for(int i=1;i<=1000;++i) 
        if(a[i].val<a[i-1].val && a[i].val < a[i + 1].val) 
        {
            pos=i,lpos=a[i-1].pos,rpos=a[i+1].pos;
            break;
        }

    now=lpos,d=(rpos - lpos)/1000;
    for(int i=1;i<=1000;++i)  
    {
        
        a[i]={now,cal(now)};
        now += d;
    }

    for(int i=1;i<=1000;++i) 
        if(a[i].val<a[i-1].val && a[i].val < a[i + 1].val) 
        {
            pos=i,lpos=a[i-1].pos,rpos=a[i+1].pos;
            break;
        }

    now=lpos,d=(rpos - lpos)/1000;
    for(int i=1;i<=1000;++i)  
    {
        a[i]={now,cal(now)};
        now += d;
    }

    for(int i=1;i<=1000;++i) 
        if(a[i].val<a[i-1].val && a[i].val < a[i + 1].val) 
        {
            pos=i,lpos=a[i-1].pos,rpos=a[i+1].pos;
            break;
        }


     now=lpos,d=(rpos - lpos)/1000;
    for(int i=1;i<=1000;++i)  
    {
        a[i]={now,cal(now)};
        now += d;
    }

    for(int i=1;i<=1000;++i) 
        if(a[i].val<a[i-1].val && a[i].val < a[i + 1].val) 
        {
            pos=i,lpos=a[i-1].pos,rpos=a[i+1].pos;
            break;
        }

   // cout<< (a[pos].val)*1.00000000000/2.000000000000 <<'\n';
   printf("%.10lf",a[pos].val*1.000000/2);
}

int main()
{
    //ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
    int _ = 1; // cin>>_;
    while (_--) solve();
    return 0;
}