The number of cars sold don't come out correct when debugging.

Option Explicit On      
Option Infer On
Option Strict Off


Public Class main

    Private Sub Button5_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button5.Click

        Static cars1 As Integer
        Static cars2 As Integer
        Static cars3 As Integer
        Static cars4 As Integer

        Dim txtID As String

        Dim txtNumberSold As Integer

        txtID = ID.Text

        txtNumberSold = convertintx(NumberSold.Text)



        If txtID.Substring(0, 1) = "F" Or txtID.Substring(0, 1) = "f" Then
            cars1 = cars1 + txtNumberSold
            Button1.Text = cars1
        End If

        If txtID.Substring(0, 1) = "P" Or txtID.Substring(0, 1) = "p" Then
            cars2 = cars2 + txtNumberSold
            Button2.Text = cars2
        End If


        If txtID.Substring(3, 1) = "1" Then
            cars3 = cars3 + txtNumberSold
            Button3.Text = cars3
        End If

        If txtID.Substring(3, 1) = "2" Then
            cars4 = cars4 + txtNumberSold
            Button4.Text = cars4
        End If



        ID.Text = ""
        NumberSold.Text = ""


    End Sub

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Button1.Text = "0"
        Button2.Text = "0"
        Button3.Text = "0"
        Button4.Text = "0"
    End Sub

    Private Function convertintx(ByVal strx As String)

        If strx = "" Then
            strx = "0"
        End If


        Dim s As String
        s = strx
        Dim str(10) As String
        Dim number(10) As Integer
        Dim sum As Integer
        sum = 0


        str = Split(s, "+")
        Dim i As Integer
        For i = 0 To str.Length - 1
            number(i) = CInt(str(i))
            sum += number(i)


        Next


        Return sum
    End Function