#include<bits/stdc++.h>
#define EPS (1e-6)
#define rep(i,n)for(int i=0;i<n;i++)
using namespace std;

int main() {
	double vl, vr, d, w; cin >> vl >> vr >> d >> w;
	bool b = true;
	double ans = 0;
	while (d > EPS) {
		double t;
		if (b)t = d / (vr + w);
		else t = d / (vl + w);
		ans += t*w;
		d -= t*(vl + vr);
		b = !b;
	}
	printf("%.7lf\n", ans);
}