Ввести числа в двух TextBox, произвести вычисления. Результат показать в третьем

Public Class Form1
    Dim a As Double
    Dim b As Double
    Public per As Double
    Public vtor As Double
    Public oper As Integer
    Public res As Double
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        If (a = 0) Then
            TextBox1.Text = TextBox1.Text & Button1.Text
        ElseIf (a = 1) Then
            TextBox2.Text = TextBox2.Text & Button1.Text
        End If
    End Sub
 
    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        If (a = 0) Then
            TextBox1.Text = TextBox1.Text & Button2.Text
        ElseIf (a = 1) Then
            TextBox2.Text = TextBox2.Text & Button2.Text
        End If
    End Sub
 
    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        If (a = 0) Then
            TextBox1.Text = TextBox1.Text & Button3.Text
        ElseIf (a = 1) Then
            TextBox2.Text = TextBox2.Text & Button3.Text
        End If
    End Sub
 
    Private Sub Button4_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button4.Click
        If (a = 0) Then
            TextBox1.Text = TextBox1.Text & Button4.Text
        ElseIf (a = 1) Then
            TextBox2.Text = TextBox2.Text & Button4.Text
        End If
    End Sub
 
    Private Sub Button5_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button5.Click
        If (a = 0) Then
            TextBox1.Text = TextBox1.Text & Button5.Text
        ElseIf (a = 1) Then
            TextBox2.Text = TextBox2.Text & Button5.Text
        End If
    End Sub
 
    Private Sub Button6_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button6.Click
        If (a = 0) Then
            TextBox1.Text = TextBox1.Text & Button6.Text
        ElseIf (a = 1) Then
            TextBox2.Text = TextBox2.Text & Button6.Text
        End If
    End Sub
 
    Private Sub Button7_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button7.Click
        If (a = 0) Then
            TextBox1.Text = TextBox1.Text & Button7.Text
        ElseIf (a = 1) Then
            TextBox2.Text = TextBox2.Text & Button7.Text
        End If
    End Sub
 
    Private Sub Button8_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button8.Click
        If (a = 0) Then
            TextBox1.Text = TextBox1.Text & Button8.Text
        ElseIf (a = 1) Then
            TextBox2.Text = TextBox2.Text & Button8.Text
        End If
    End Sub
 
    Private Sub Button9_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button9.Click
        If (a = 0) Then
            TextBox1.Text = TextBox1.Text & Button9.Text
        ElseIf (a = 1) Then
            TextBox2.Text = TextBox2.Text & Button9.Text
        End If
    End Sub
 
    Private Sub Button10_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button10.Click
        If (a = 0) Then
            TextBox1.Text = TextBox1.Text & Button10.Text
        Else (a = 1) Then
            TextBox2.Text = TextBox2.Text & Button10.Text
        End If
    End Sub
 
    Private Sub Button11_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button11.Click
        If (a = 0) Then
            TextBox1.Text = TextBox1.Text & Button11.Text
        ElseIf (a = 1) Then
            TextBox2.Text = TextBox2.Text & Button11.Text
        End If
    End Sub
 
    Private Sub Button12_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button12.Click  // Для кнопки плюс, пока что не получается
        a = 1
        If (a = 1) Then
            per = TextBox1.Text
          vtor = TextBox2.Text
            TextBox3.Text = "+"
            res = per + vtor
            TextBox4.Text = res
        End If
    End Sub
End Class