Calculating from inputboxes, with error message for letter input

Calculating from inputboxes, with error message for letter input

I've been struggelig with an embarrassing and annoying little problem in
visual basic (I'm, as you probbably see, a beginner). The problem is the
error message system for when letters are entered instead of numbers. I
get "can't convert from integer to string.
Any help on this would be most appreciated.
Here is my code:
Dim number1, number2 As Integer
Dim sum As String
number1 = InputBox("first value:")
number2 = InputBox("second value:")
sum = number1 + number2
If IsNumeric(sum) Then
MsgBox("The sum of the numbers " & number1 & " and " & number2 & "
is: " & sum)
ElseIf Not IsNumeric(sum) Then
MsgBox("You may only type numbers into the fields!, trie again")
End If
In advance, thank you :)!