import java.util.*;
class B{
	static Scanner s = new Scanner(System.in);
	public static void main(String[] args) {
		int
			vl=s.nextInt(),
			vr=s.nextInt(),
			d =s.nextInt(),
			w =s.nextInt();

		double
			xl=0,
			xr=d,
			r=0;

		for(int i=0;i<11451419;++i){

			double t=(i%2==0)?
					(xr-xl)/(w+vr):
					(xr-xl)/(w+vl);
			r+=t*w;
			xl+=t*vl;
			xr-=t*vr;
		}
		System.out.printf("%.20f\n",r);
	}
}