Visual Web Developer 2010 Express
This is an ASP.net project

I have searched but wasn't successful

I have two text boxes and I need to make small calculator.
I have made one "IF" for checking is there something in textboxes, if they are empty then I get message that I didn't insert anything.

And if I insert numbers then I get the result.
This is ok.

I have a problem with validation

I need ONLY numbers to be inserted, if I insert something that is not integer I need a message that this is not a number.

Please, hlp.

This is my code :

Code:
Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button1.Click

        If TextBox1.Text = ("") Or TextBox2.Text = ("") Then
            Label5.Text = ("Niste unjeli dva broja, pokusajte ponovo")
        Else
            Dim prvi As Integer = TextBox1.Text
            Dim drugi As Integer = TextBox2.Text
            Label5.Text = (prvi + drugi)
        End If

    End Sub