import java.util.Scanner;

public class Test {
	public static Scanner sc = new Scanner(System.in);

	public static void main(String[] args) {
		double ax = sc.nextDouble(), ay = sc.nextDouble(), bx = sc.nextDouble(), by = sc.nextDouble();
		double ty = ay-by, tx = bx/(ax+bx);
		System.out.println(by+tx*ty);
	}
}