#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)

int main(){
    long double x,y,z,w;
    cin>>x>>y>>z>>w;
    long double ad=y+w;
    long double cd=x/ad;
    long double dq=cd-z;
    long double ans=x-y*cd/2-w*dq/2-z*ad/2;
    //cout<<y*cd/2<<" "<<w*dq/2<<" "<<z*ad/2<<endl;
    cout<<fixed<<setprecision(12)<<ans<<endl;
    return 0;
}