Visual Basic Input Box Loop if nohting - vba

Question

I made a coding about calculation, there are 2 numbers in the 2 different 2 boxes, when I calculate the answer is wrong, there is a msgbox shown "Try again", if correct, the msgbox shown "you are correct", but is insert nothing or words then press enter, it will be shown error.
I want if the inputbox insert nothing then press enter, the inputbox will be shown again to restrick someone insert something into the inputbox and can not insert any words into the inputbox, if insert any words, also the inputbox will be shown again return to empty.
Does anyone can tell me how to solve this problem?
Thank you so much.
Dim a As String
Do While True
a = InputBox("Please enter your answer")
If a = Val(txtnumber1.Text) + Val(txtnumber2.Text) Then
Exit Do
Else
MsgBox("Try again!!!!")
End If
Loop
MsgBox("You are correct!")
End Sub
End Class

Answer 1

Dim a As String
Do While True
Do While a="" Or Not IsNumeric(a)
a = InputBox("Please enter your answer")
Done
If val(a) = Val(txtnumber1.Text) + Val(txtnumber2.Text) Then
Exit Do
Else
MsgBox("Try again!!!!")
End If
Loop
MsgBox("You are correct!")

Answer 2

InputBox in Do...Loop
Sub QnA()
Const Title As String = "Q&A"
Dim Answer As String
Dim TryAgain As Long
Do
Answer = InputBox("Please enter your answer", Title, "")
If Len(Answer) = 0 Then
MsgBox "Nothing entered.", vbExclamation, Title
Exit Sub
End If
If IsNumeric(Answer) Then
If Val(Answer) = Val(txtnumber1.Text) + Val(txtnumber2.Text) Then
Exit Do
End If
End If
TryAgain = MsgBox("Wrong answer (""" & Answer & """). Try again?", _
vbYesNo + vbQuestion, Title)
If TryAgain = vbNo Then Exit Sub
Loop
MsgBox "You are correct!", vbInformation, Title
End Sub