import java.util.*;
import java.io.*;

public class Main {
    public static void main(String[] args) throws Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String[] first = br.readLine().split(" ", 2);
        double xa = Integer.parseInt(first[0]);
        double ya = Integer.parseInt(first[1]);
        String[] second = br.readLine().split(" ", 2);
        double xb = Integer.parseInt(second[0]);
        double yb = Integer.parseInt(second[1]);
        double ans = ya + xa / (xa + xb) * (yb - ya);
        System.out.println(ans);
    }
}