#include <bits/stdc++.h>

using namespace std;
using LL = long long;
using ULL = unsigned long long;
using VI = vector<int>;
using VL = vector<LL>;
using PII = pair<int, int>;
using PLL = pair<LL, LL>;
using ld = long double;

#define SZ(a) (int)a.size()
#define ALL(a) a.begin(), a.end()
#define MP make_pair
#define PB push_back
#define EB emplace_back
#define F first
#define S second
#define FOR(i, a, b) for (int i = (a); i<(b); ++i)
#define RFOR(i, b, a) for (int i = (b)-1; i>=(a); --i)
#define FILL(a, b) memset(a, b, sizeof(a))

typedef long double db;

db h, r, da, dw;

db f(db x) {
	return (h * da * (h - x) / (x * (dw - da) + da * h) + x) / 2;
}

int main()
{
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout << fixed << setprecision(10);
	cin >> h >> r >> da >> dw;
	double low = 0, high = h;
	FOR(it, 0, 747) {
		double mid1 = low + (high - low) / 3, mid2 = high - (high - low) / 3;
		if (f(mid1) < f(mid2)) {
			high = mid2;
		}
		else {
			low = mid1;
		}
	}
	cout << (low + high) / 2 << "\n";
}