#include <bits/stdc++.h>
using namespace std;
const int maxn = 6000105;
const int maxm = 6000005;
typedef vector<int> vi;
typedef pair<int, int> pii;
const int inf = 0X3f3f3f3f;
#define debug(x) cout<<#x<<':'<<x<<endl;

typedef long double ld;

const ld pi=acos(-1.0);

//const int maxn=1e5+100;
const ld eps=1e-9;
ld h,r,da,dw;

ld check(ld a){
    
    // double ans1=0.5*mid*(1-da/dw)+0.5*h*(da/dw);
    // double ans2=0.5*h-mid/2*(dw/da-1);
    // if(ans1<=mid) return ans1;
    // else if(ans2>=mid) return ans2;
    // else {
    //     cout<<"****"<<endl;
    //     return 0;
    // }
    

    // ld ans=0.5*h*(mid+h)*da/(mid*dw+mid*3*da+h*da);
    // //ld ans2=
    // ans=ans*h/2+mid/2;
    // ld ans2=(a+(h-a)/2)*da*h/2;
    // ld ansx=a/2*dw+(a+(h-a)/2)*da;
    // ld anss=ans2/ansx+a/2;

    ld anss=(h-a)*da/(a*dw+(h-a)*da)*h/2+a/2;
    return anss;

    
}
int main(){
    
    cin>>h>>r>>da>>dw;

    ld l=0,r=h;
    while(r-l>eps){
        ld mid1=(2*l+r)/3;
        ld mid2=(l+2*r)/3;

        if(check(mid1)<check(mid2)){
            r=mid2;
        }
        else {
            l=mid1;
        }
    }

    cout<<fixed<<setprecision(8)<<l<<endl;
    //cout<<l<<endl;
}