#include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> PII;
#define int long long

double heart(double h, double x, double pw, double pa)
{
	return 0.5 * (pw * x * x + pa * (h - x) * (h - x)) / (pw * x + pa * (h - x));
}

double search(double h, double pw, double pa)
{
	double head = 0;
	double tail = h;
	while ((tail - head) > 1e-6) 
	{
		double mid = (head + tail) / 2.0;
		if (heart(h, mid, pw, pa) < heart(h, mid + 1e-6, pw, pa))
		{
			tail = mid;
		} 
		else  
		{
			head = mid;
		}
	}
	return head;
}

signed main()
{
	ios::sync_with_stdio(false);
    cin.tie(0);cout.tie(0);
	double h, pw, pa, r;
	cin >> h >> r >> pa >> pw;
	cout << fixed << setprecision(10) << search(h, pw, pa);
}