#include<iostream>
#include<vector>
#include<string>
#include<cstring>
#include<algorithm>
#include<map>
#include<set>
#include<cmath>
#include<cassert>
#include<queue>

using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef vector<int> vi;

int main() {
	double vl, vr;
	double d;
	double w;
	cin >> vl >> vr;
	cin >> d;
	cin >> w;
	double t1 = d/(w+vr);
	double d1 = d-t1*(vl+vr);
	double t2 = d1/(vl+w);
	double d2 = d1-t2*(vl+vr);
	double ans = (t1+t2)*w / (1-d2/d);
	printf("%.10lf\n", ans);
	return 0;
}