Public Class Form1
    Dim broj1, broj2 As Integer

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        If Len(TextBox1.Text) < 1 Or Len(TextBox2.Text) < 1 Then
            MsgBox("Unesi broj!")
        Else
            Label1.Text = broj1 + broj2
        End If
    End Sub

    Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged
        If IsNumeric(TextBox1.Text) Then
            broj1 = CInt(TextBox1.Text)
        Else
            MsgBox("Unesi broj!")
        End If
    End Sub

    Private Sub TextBox2_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox2.TextChanged
        If IsNumeric(TextBox2.Text) Then
            broj2 = CInt(TextBox2.Text)
        Else
            MsgBox("Unesi broj!")
        End If
    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        If Len(TextBox1.Text) < 1 Or Len(TextBox2.Text) < 1 Then
            MsgBox("Unesi broj!")
        Else
            Label1.Text = broj1 - broj2
        End If
    End Sub

    Private Sub Button4_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button4.Click
        If Len(TextBox1.Text) < 1 Or Len(TextBox2.Text) < 1 Then
            MsgBox("Unesi broj!")
        Else
            Label1.Text = broj1 / broj2
        End If
    End Sub

    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        If Len(TextBox1.Text) < 1 Or Len(TextBox2.Text) < 1 Then
            MsgBox("Unesi broj!")
        Else
            Label1.Text = broj1 * broj2
        End If
    End Sub
End Class