#include<stdio.h>

int main(void){
	double vl,vr,d,w,k1,k2,k3,k4,k5;
	scanf("%lf%lf%lf%lf",&vl,&vr,&d,&w);
	k1 = w/(w+vr);
	k2 = w/(w+vl);
	k3 = k1 - ((1-k1)*vl/vr);
	k4 = k2 - ((1-k2)*vr/vl);
	k5 = k3*k4;
	k5 = 1/k5;
	k5 = k5 - 1;
	k5 = 1/k5;
	k5 = k5 + 1;
	printf("%.20lf",k5*d*(k1 + k3*k2));
	return 0;
}