Module _20190421

    Sub Main()

        Dim sRead1 As String = Console.ReadLine()
        Dim sRead2 As String = Console.ReadLine()

        Dim iColumn As Integer = sRead1.Split(" ")(0)
        Dim iRow As Integer = sRead1.Split(" ")(1)

        Dim iBlackColumn As Integer = sRead2.Split(" ")(0)
        Dim iBlackRow As Integer = sRead2.Split(" ")(1)

        Console.WriteLine(iColumn * iRow - iBlackColumn * iRow - iBlackRow * iColumn + iBlackColumn * iBlackRow)

    End Sub

End Module