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

int main()
{
	double v1,v2,d,w;
	cin>>v1>>v2>>d>>w;
	double a=w*d/(w+v2);
	double r1=(w-v1)/(w+v1);
	double r2=(w-v2)/(w+v2);
	cout<<fixed<<setprecision(8)<<a*(r1+1)/(1-r1*r2)<<endl;
	return 0;
}