Help me finish my code

Dim strTextBox As String
        Dim decItemPrice As Decimal
        Dim decSubTotal As Decimal
        Dim intNumber As Integer
        Dim decSalesTax As Decimal
        Dim decTotalSale As Decimal

        Do

            strTextBox = InputBox("Enter item price", "Item Price")

            If Decimal.TryParse(strTextBox, decItemPrice) Then

                decItemPrice = Convert.ToDecimal(strTextBox)
                decSubTotal = decItemPrice
                intNumber = intNumber + 1
                txtItemPrice.Text = FormatCurrency(decItemPrice)
                txtSubTotal.Text = FormatCurrency(decSubTotal)
                txtNumber.Text = Convert.ToString(intNumber)

            End If

        Loop Until strTextBox = ""

        decSalesTax = 0.065D * decSubTotal
        decTotalSale = decSubTotal + decSalesTax
        txtSalesTax.Text = FormatCurrency(decSalesTax)
        txtTotalSale.Text = FormatCurrency(decTotalSale)

    End Sub

I need help doing two things: 1) Making sure only numerical data can be entered or else a message box is displayed. 2) Keep a running Sub Total so that the first entered number is added to the second entered and so on until the loop is terminated.

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YOU JUST LOST THE GAME