import java.util.Scanner;

public class No306 {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		double xa = sc.nextInt();
		double ya = sc.nextInt();
		double xb = sc.nextInt();
		double yb = sc.nextInt();
		
		System.out.println(yb - (yb - ya)/(xb + xa)*xb);
	}
}