#include <iostream>
#include <cstdio>
using namespace std;

double vl, vr;
double d;
double w;

int main() {
	cin >> vl >> vr >> d >> w;
	
	double t1 = d / (vr + w);
	double t2 = (d - t1 * (vl + vr)) / (vl + w);
	
	double dist = (t1 + t2) * w;
	double radio = (d - (t1 + t2) * (vl + vr)) / d;
	
	//dist + radio * dist + radio^2 * dist + … = dist / (1 - radio)
	printf("%.14f\n", dist / (1 - radio));
	
	return 0;
}