#include <iostream>
#include <iomanip>
#include <cstdlib>
using namespace std;

int main()
{
	double vl, vr, d, w; cin >> vl >> vr >> d >> w;

	double dist = 0;

	double l = 0, r = d;
	while (l < r - 0.00000001) {
		double cnt;
		if (w > 0) cnt = (r - l) / (w + vr);
		else cnt = (r - l) / (vl - w);
		dist += cnt*abs(w);
		l += cnt*vl;
		r -= cnt*vr;
		w = -w;
	}

	cout << fixed << setprecision(8) << dist << endl;

	return 0;
}