import java.util.Scanner;

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