#include <bits/stdc++.h>
template<class T> inline bool chmin(T&a, T b){if(a > b){a = b; return true;}else{return false;}}
template<class T> inline bool chmax(T&a, T b){if(a < b){a = b; return true;}else{return false;}}
#define ll long long
#define double long double
#define rep(i,n) for(int i=0;i<(n);i++)
#define REP(i,n) for(int i=1;i<=(n);i++)
#define mod (ll)(1e9+7)
#define inf (ll)(3e18+7)
#define eps (double)(1e-9)
#define pi (double) acos(-1)
#define P pair<ll,ll>
#define PiP pair<int,pair<int,int>>
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
using namespace std;

double x, y, z, w;
double f(double n){
    return abs(sqrtl(n*n+y*y) - sqrtl((x/n)*(x/2)+z*z) - w);
}

int main() {
    cin >> x >> y >> z >> w;
    double low = y, high = x;
    rep(_, 500){
        double c1 = (low * 2 + high) / 3;
        double c2 = (low + high * 2) / 3;
        if(f(c1) > f(c2))low = c1;
        else high = c2;
    }
    double a = high;
    double b = x/a;
    cout << fixed << setprecision(8) << x-(b*y+(a-y)*(b-z)+a*z)/2 << endl;
}