Imports System

'9780553173253          

Public Module Module1

    Public Sub Main()

        Dim isbn = ""

        Dim total = 0

        Dim checkdigit = 0

        do 

            console.WriteLine("Enter ISBN:")

            isbn = console.ReadLine()

        loop until isbn.Length = 13

​

        For i = 0 To isbn.Length - 3 step 2

            total = total + CInt(isbn.Substring(i, 1))

        Next i

        for i = 1 to isbn.Length - 2 step 2

            total = total + 3*CInt(isbn.Substring(i, 1))

        next i

        checkdigit = 10 - (total MOD 10)

        if checkdigit = Cint(isbn.Substring(12,1)) then

            Console.WriteLine("ISBN is valid")  

        else

            Console.WriteLine("ISBN is invalid")    

        end if

    End Sub

End Module