#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) begin(v),end(v)
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
using ll = long long;
using pii = pair<int, int>;
constexpr ll INF = 1ll<<30;
constexpr ll longINF = 1ll<<60;
constexpr ll MOD = 1000000007;
constexpr bool debug = false;
//---------------------------------//

int main() {
	int X, Y, Z, W;
	cin >> X >> Y >> Z >> W;
	double l = Z, r = (double)X / Y;
	REP(i, 50) {
		double m = (l + r) / 2; // w = m
		double h = X / m;
		double qb = hypot(h, Z);
		double bp = hypot(Y, m);
		if (qb + W <= bp) r = m;
		else l = m;
	}
	double h = r, w = X / r;
	double ans = X;
	ans -= (w - Z) * (h - Y) / 2;
	ans -= Y * w / 2;
	ans -= Z * h / 2;
	printf("%.16f\n", ans);
}