package no305;

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		double xa = sc.nextDouble();
		double ya = sc.nextDouble();
		double xb = sc.nextDouble();
		double yb = sc.nextDouble();
		System.out.println(ya + (yb - ya) * xa / (xa + xb));

	}

}