#include<iostream>
#include<algorithm>
#include<iomanip>
using namespace std;
const double EPS=1e-5;

int main()
{
	double v1,v2,d,w;
	cin>>v1>>v2>>d>>w;
	double sum=0.0;
	double a=w*d/(w+v2);
	sum+=a;
	double r1=(w-v1)/(w+v1);
	double r2=(w-v2)/(w+v2);
	int count=1;
	while(a>EPS)
	{
		if(count%2==0)a*=r2;
		else a*=r1;
		sum+=a;
		count++;
	}
	cout<<fixed<<setprecision(8)<<sum<<endl;
	return 0;
}