using System;

public class Program

{

    static private double slope(double x1, double x2, double y1, double y2)

    {   

        double ans1 = x1 - x2 / y1 - y2;

        return ans1;

    }

    public static void Main()

    {

        start:

double input1 = double.Parse(Console.ReadLine());

        double input2 = double.Parse(Console.ReadLine());

        double input3 = double.Parse(Console.ReadLine());

        double input4 = double.Parse(Console.ReadLine());

        Console.WriteLine(slope(input1, input2, input3, input4));

        goto start;

    }

}