#include <bits/stdc++.h>
//#include <chrono>
//#pragma GCC optimize("Ofast")
using namespace std;
#define reps(i,s,n) for(int i = s; i < n; i++)
#define rep(i,n) reps(i,0,n)
#define Rreps(i,n,e) for(int i = n - 1; i >= e; --i)
#define Rrep(i,n) Rreps(i,n,0)
#define ALL(a) a.begin(), a.end()

using ll = long long;
using vec = vector<ll>;
using mat = vector<vec>;

ll N,M,H,W,Q,K,A,B;
string S;
using P = pair<ll, ll>;
const ll INF = (1LL<<60);

template<class T> bool chmin(T &a, const T b){
    if(a > b) {a = b; return true;}
    else return false;
}
template<class T> bool chmax(T &a, const T b){
    if(a < b) {a = b; return true;}
    else return false;
}
template<class T> void my_printv(std::vector<T> v,bool endline = true){
    if(!v.empty()){
        for(std::size_t i{}; i<v.size()-1; ++i) std::cout<<v[i]<<" ";
        std::cout<<v.back();
    }
    if(endline) std::cout<<std::endl;
}

long double x, y, z, w, lb = 1.0, ub = 1e+6 + 1;
long double menseki(long double yoko){
    long double bq = sqrt(z * z + yoko * yoko);
    long double bp = bq + w;
    long double tate = sqrt(bp * bp - y * y);
    return tate * yoko;
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cin>>x>>y>>z>>w;
    while(ub - lb > 1e-11){
        long double cen = (ub + lb) / 2;
        (menseki(cen) <= x ? lb : ub) = cen;
    }
    long double res = x - (lb * z + (x / lb) * y + (lb - y) * (x / lb - z)) / 2.0;
    cout<<fixed<<setprecision(15)<<res<<endl;
}