How to Increment datarow by value 10

Public Class frmVisitor
 
    Private dbConnection As New OleDb.OleDbConnection()
    'To add a DataAdapter
    Private daVisitor As OleDb.OleDbDataAdapter
    'To add a CommandBuilder
    Private cmdbVisitor As OleDb.OleDbCommandBuilder Private dtVisitor As New DataTable
    Private rpVisitor As Integer = 0
    Dim intNewVisitorID As Integer
 
    Private Sub frmVisitor_FormClosed(ByVal sender As Object,  ByVal e As System.Windows.Forms.FormClosedEventArgs Handles Me.FormClosed
 
        'Closing a Connection to a Data Source ta the time when Main Form Closed
        'this is accomplished by calling the Close() method of the connection object
        dbConnection.Close()
        dbConnection.Dispose()
    End Sub
 
    Private Sub frmVisitor_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
 
        dbConnection.ConnectionString = _
        "Provider=Microsoft.Jet.OLEDB.4.0;Data Source =SNGPLDbase.mdb"
 
         daVisitor = New OleDb.OleDbDataAdapter("Select * From Visitor", dbConnection)
        'To initialize the CommandBuilder object
        cmdbVisitor = New OleDb.OleDbCommandBuilder(daVisitor)
 
        daVisitor.Fill(dtVisitor) 'Because the DataTable doesn't hold a connection to the data source, you don't need to close it when you're finished.
        'Used to display the current record in the data table
        'Me.ShowVisitorRecord()
    End Sub
 
    Private Sub ShowVisitorRecord()
 
        If dtVisitor.Rows.Count = 0 Then 'When there is no record
 
            Me.cboVisitorCode.Text = ""
            Me.txtVisitorName.Text = ""
            Me.txtVehicleNo.Text = ""
            Exit Sub
 
        End If  'When there is record
 
        cboVisitorCode.Text = dtVisitor.Rows(rpVisitor)("VisitorCode")
        txtVisitorName.Text = dtVisitor.Rows(rpVisitor)("VisitorName")
        txtVehicleNo.Text = dtVisitor.Rows(rpVisitor)("VehicleNo")
 
    End Sub
 
   Private Sub btnAdd_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnAdd.Click
 
        If dtVisitor.Rows.Count = 0 Then 'When there is no record
 
            Dim intNewVisitorID = 10 'Initial value of VisitorCode
            Dim drNewRow As DataRow = dtVisitor.NewRow()
            drNewRow.Item("VisitorCode") = intNewVisitorID
 
            Me.cboVisitorCode.Items.Add(drNewRow.Item("VisitorCode"))
            Me.cboVisitorCode.Text = drNewRow.Item("VisitorCode")
 
            '  cboVisitorCode.Text = dtVisitor.Rows(rpVisitor)("VisitorCode")
            dtVisitor.Rows.Add(drNewRow)
 
            Exit Sub
        Else
 
            ' If there are any rows in the data table, 
            ' move to the last and show the record.
 
            If dtVisitor.Rows.Count > 0 Then
                rpVisitor = dtVisitor.Rows.Count - 1
                ' Me.ShowVisitorRecord()
            End If
 
            Dim drNewRow As DataRow = dtVisitor.NewRow()
            intNewVisitorID = drNewRow.Item("VisitorCode") + 10 'VisitorCode increase by 10
            drNewRow.Item("VisitorCode") = intNewVisitorID
 
            'To Insert new VisitorCode to Combobox In case of New Entry.
            Me.cboVisitorCode.Items.Add(drNewRow.Item("VisitorCode"))
            Me.cboVisitorCode.Text = drNewRow.Item("VisitorCode")
            dtVisitor.Rows.Add(drNewRow)
 
        End If
    End Sub
End Class