// Enjoy your stay. Code by evima

#include <bits/stdc++.h>

#define LOOPVAR_TYPE long long

#define all(x) (x).begin(), (x).end()
#define sz(x) ((LOOPVAR_TYPE)(x).size())
#define foreach(it, X) for(__typeof((X).begin()) it = (X).begin(); it != (X).end(); it++)
#define GET_MACRO(_1, _2, _3, NAME, ...) NAME
#define _rep(i, n) _rep2(i, 0, n)
#define _rep2(i, a, b) for(LOOPVAR_TYPE i = (LOOPVAR_TYPE)(a); i < (LOOPVAR_TYPE)(b); i++)
#define rep(...) GET_MACRO(__VA_ARGS__, _rep2, _rep)(__VA_ARGS__)

template<class T> bool chmin(T &a, const T &b) { return (b < a) ? (a = b, true) : false; }
template<class T> bool chmax(T &a, const T &b) { return (a < b) ? (a = b, true) : false; }

#define fir first
#define sec second
#define mp make_pair
#define mt make_tuple
#define pb push_back
#define eb emplace_back

const double EPS = 1e-9;
const double PI = 3.141592653589793238462;
const long long INF = 1070000000LL;
const long long MOD = 998244353LL;// 1000000007LL;

using namespace std;
using ld = long double;
using ll = long long;

ld X, Y, Z, W;

int main() {
    cin.tie(NULL);
    ios_base::sync_with_stdio(false);
    
    cin >> X >> Y >> Z >> W;
    ld lo = Z, hi = 1e18, ans;
    rep(t, 100){
        ld mi = (lo + hi) / 2;  // BQ
        ld BC = sqrt(mi * mi - Z * Z);
        ld BP = BC + W;
        ld AB = sqrt(BP * BP - Y * Y);
        (AB * BC < X ? lo : hi) = mi;
        if(t == 99){
            ans = X - BC * Z / 2 - AB * Y / 2 - (AB - Z) * (BC - Y) / 2;
        }
    }
    cout << setprecision(17) << ans << endl;
}