import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		double x1 = sc.nextDouble();
		double y1 = sc.nextDouble();
		double x2 = sc.nextDouble();
		double y2 = sc.nextDouble();

		System.out.println((y2 - y1) / (x2 + x1) * x1 + y1);
	}
}