#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <iomanip>
using namespace std;

int main()
{
	double vl, vr, d, w;
	cin >> vl >> vr >> d >> w;
	cout <<fixed<<setprecision(7)<< d/(vl + vr)*w << endl;
    return 0;
}