import java.util.Scanner;
public class Main{
	public static void main(String... args){
		Scanner scan = new Scanner(System.in);
		System.out.println(calc(scan));
	}

	public static double calc(Scanner scan){
		double xa = scan.nextDouble();
		double ya = scan.nextDouble();
		double xb = scan.nextDouble();
		double yb = scan.nextDouble();

		if(ya == yb) return ya;
		if(xa == 0 && xb == 0) return (ya+yb)/2.0;

		return yb-(yb-ya)/(xb+xa)*xb;
	}
}