import java.util.*;

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