Accepts An 8 Digit Hex Encrypted String And Adds It To A Listbox1?

I have a text box that accepts an 8 digit hex encrypted string and adds it to a listbox1. Then I have a button that when clicked takes the string from listbox1 and

1) reverses the string

2) subtracts 235

3) Converts it back to a 9 digit number

4) Adds it to listbox2 using the

Private Sub decrypt_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles decrypt.Click
Dim i As Integer
Dim buffer As String = ListBox1.Items(i)[code].....

However something seems to be wrong because it get an arithmetic overflow exception.