#include<bits/stdc++.h>
#define INF 200010
#define mod 80112002
#define pi 3.14159265359
using namespace std;
typedef long long LL;
double h,r,pa,pw;
double check(double x)
{
    return (x*x*pw+(h-x)*(h+x)*pa)/(x*pw+(h-x)*pa)*0.5;
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    cin>>h>>r>>pa>>pw;
    double l=0,r=h;
    while((r-l)>1e-7)
    {
        double mid=(l+r)/2;
        if(check(mid)<check(mid+1e-7))
            r=mid;
        else
            l=mid;
    }
    cout<<fixed<<setprecision(10)<<(l+r)/2;
    return 0;
}