#include "bits/stdc++.h"
using namespace std;
#define int long long
using ll = long long;
using P = pair<ll, ll>;
const ll INF = (1LL << 61);
ll mod = (ll)1e9 + 7;

ll X, Y, Z, W;
bool f(long double mid) {
	long double now = sqrt((X / mid) * (X / mid) + Z * Z) + W;
	long double now2 = sqrt(mid * mid + Y * Y);
	if (now < now2)return true;
	else return false;
}

signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cin >> X >> Y >> Z >> W;
	long double l = 1, r = 1e9;
	for (int i = 0; i < 200; i++) {
		long double mid = (l + r) / 2;
		if (f(mid))r = mid;
		else l = mid;
	}
	long double ans = X;
	ans -= (X / l) * Z / 2.0;
	ans -= ((X / l) - Y) * (l - Z) / 2.0;
	ans -= l * Y / 2.0;
	cout << fixed << setprecision(15) << ans << endl;
	return 0;
}