using System;

public class Program

{

    public static void Main()

    {

        int a, b, c, d;

        a = int.Parse(Console.ReadLine());

        b = int.Parse(Console.ReadLine());

        c = int.Parse(Console.ReadLine());

        d = int.Parse(Console.ReadLine());

        int difference = (a * b) - (c * d); // difference between a and b to c and d

        Console.WriteLine();

        Console.WriteLine("Difference = " + difference);

    }

}