Imports System.Collections.Generic
Imports System.Console
Imports System.Math  
Module Program
 	Sub Main()
        Dim input() As String = Console.ReadLine().Split(" ")
        Dim H As Integer = input(0)
        Dim W As Integer = input(1)
        Dim inputB() As String = Console.ReadLine().Split(" ")
        Dim h2 As Integer = inputB(0)
        Dim w2 As Integer = inputB(1)
        Console.WriteLine((H-h2)*(W-w2))
    End Sub
End Module
