Public Class Form1

    Private Sub btnAdd_Click(sender As Object, e As EventArgs) Handles btnAdd.Click
        lstNumbers.Items.Add(CInt(txtAdd.Text))
    End Sub

    Private Sub btnSearch_Click(sender As Object, e As EventArgs) Handles btnSearch.Click
        Dim flag As Boolean = False
        Dim first As Integer = 0
        Dim last As Integer = lstNumbers.Items.Count - 1
        Dim middle As Integer
        Dim query As Integer = CInt(txtQuery.Text)

        Do Until first > last
            middle = CInt((first + last) / 2)

            If CInt(lstNumbers.Items.Item(middle)) = query Then
                flag = True
            End If

            If CInt(lstNumbers.Items.Item(middle)) > query Then
                last = middle - 1
            Else
                first = middle + 1
            End If
        Loop

        If flag Then
            txtOutput.Text = "The number is in the list"
        Else
            txtOutput.Text = "The number is NOT in the list"
        End If
    End Sub

End Class