import java.util.Date;
import java.util.Scanner;
public class main{
	public static void main(String[] args){

		Scanner sc=new Scanner(System.in);
		long exec_time=new Date().getTime();
		double x_a=sc.nextDouble();
		double y_a=sc.nextDouble();
		double x_b=sc.nextDouble();
		double y_b=sc.nextDouble();
		System.out.println(y_b-(y_b-y_a)*x_b/(x_b+x_a));
		System.err.println(new Date().getTime()-exec_time+"ms");
	}
}