Get maximum itemid from nested observablecollection

Hi

I have nested observablecollection. It contains n levels.

How to get maximum id (integer type) from this nested observablecollection, as i have to add new item at the selected node.

In order to add new item into nested observablecollection, i require the id of last item in collection.

Following is the Observablecollection:

Imports System.Collections.ObjectModel
Imports Telerik.Windows.Controls
Imports System.ComponentModel

Public Class BefundItem
    Implements INotifyPropertyChanged

    Public Event PropertyChanged As PropertyChangedEventHandler _
          Implements INotifyPropertyChanged.PropertyChanged

    Private Sub NotifyPropertyChanged(ByVal info As String)
        RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(info))
    End Sub

    Public Sub New(Id As String, ParentId As String, text As String)
        Me.Id = Id
        Me.ParentId = ParentId
        Me.Text = text
        Me.Items = New ObservableCollection(Of BefundItem)()
    End Sub

    Private _Id As Integer
    Public Property Id() As Integer
        Get
            Return _Id
        End Get
        Set(ByVal value As Integer)
            _Id = value
            NotifyPropertyChanged("Id")
        End Set
    End Property

    Private _ParentId As Integer
    Public Property ParentId() As Integer
        Get
            Return _ParentId
        End Get
        Set(ByVal value As Integer)
            _ParentId = value
            NotifyPropertyChanged("ParentId")
        End Set
    End Property

    Private _Text As String
    Public Property Text() As String
        Get
            Return _Text
        End Get
        Set(ByVal value As String)
            _Text = value
            NotifyPropertyChanged("Text")
        End Set
    End Property

    Public ReadOnly Property IsExpandable() As Boolean
        Get
            Return True
        End Get
    End Property

    Public m_Items As ObservableCollection(Of BefundItem)
    Public Property Items() As ObservableCollection(Of BefundItem)
        Get
            Return m_Items
        End Get
        Set(value As ObservableCollection(Of BefundItem))
            m_Items = value
            Me.NotifyPropertyChanged("Items")
        End Set
    End Property

    Public Property Count() As Integer
        Get
            Return m_Count
        End Get
        Set(value As Integer)
            m_Count = value
            Me.NotifyPropertyChanged("Count")
        End Set
    End Property
    Private m_Count As Integer


End Class

Code to fetch data from observablecollection to Treelistview:

 Private Sub Fill_Befunde_Treeview()
        Try
            For j As Integer = 1 To 2
                id = j
                Parentid = 0
                text = j

                Dim parent As New BefundItem(id, Parentid, text)

                For i As Integer = 1 To 4
                    id = i
                    Parentid = j
                    text = j & "." & i

                    Dim child As New BefundItem(id, Parentid, text)
                    parent.Items.Add(child)
                Next
                data.Add(parent)
            Next

            trvlstBefunde_Befunde.ItemsSource = data
        Catch ex As Exception
            MsgBox(ex.Message)
        End Try
    End Sub

Thank you in advance.

S.Naik