How to use large integers in Visual Basic VB without BigInteger

i want to calculate the large integer but my VB 2015 doesnt have System.Numerics

source

Module module21

'modular balbulator
'http://www.codeabbey.com/index/task_view/modular-calculator
'19/02/17

Sub main()
    Dim processed As Int64 = Console.ReadLine()
    Dim process As String
    process = Console.ReadLine()
    Do
        If process.Substring(0, 1) = "+" Then
            processed += Int64.Parse(process.Split(" ")(1))
        ElseIf process.Substring(0, 1) = "*" Then
            processed *= Int64.Parse(process.Split(" ")(1))
        End If

        process = Console.ReadLine()
    Loop Until process.Substring(0, 1) = "%"

    processed = processed Mod Int64.Parse(process.Split(" ")(1))
    Console.Write(processed)
    Console.ReadLine()
End Sub

End Module

doesnt matter now i used biginteger cuH