#include <stdio.h>
#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
#include <functional>
#include <map>
#include <iomanip>
#include <math.h> 
#include <stack>
#include <queue>
#include <bitset>
#include <cstdlib>
#include <tuple>
#include <cctype>
#include <ctype.h>
#include <set>

using namespace std;


int main() {
	int i, j, k;
	double vl, vr, d, w;
	double tvl, tvr, wavl, wavr, kyvl, kyvr;


	cin >> vl >> vr >> d >> w;

	kyvl = d * (vl / (vl + vr));
	kyvr = d - kyvl;

	tvl = kyvl / vl;
	tvr = kyvr / vr;

	cout << fixed << setprecision(9) << tvl * w << endl;











	getchar();
	getchar();
	return 0;
}