#include<bits/stdc++.h>
using namespace std;

#define ll long long

int main()
{
	cin.tie(0)->sync_with_stdio(false);
	cin.exceptions(ios::badbit | ios::failbit);
	cout << fixed << setprecision(15);
	double h, r, da, dw;
	cin >> h >> r >> da >> dw;
	auto f = [&](double x){
		return x / 2 + h / 2 * ((h - x) * da) / ((h - x) * da + x * dw);
	};
	double low = 0, high = h;
	for(auto rep = 300; rep > 0; -- rep){
		double m1 = (2 * low + high) / 3, m2 = (low + 2 * high) / 3;
		if(f(m1) < f(m2)){
			high = m2;
		}
		else{
			low = m1;
		}
	}
	cout << (low + high) / 2 << "\n";
	return 0;
}