#include <iostream>
using namespace std;

int main()
{
	double vl, vr;
	double d;
	double w;
	cin >> vl >> vr >> d >> w;
	//(vl+vr)*t=d;t=d/(vl+vr);
	printf("%f\n", d / (vl + vr)*w);
    return 0;
}