VB.net 双重循环问题

验证哥德巴赫猜想:任意一个大于等于6的偶数都可以分解为两个质数之和。
基本思想：n为大于等于6的任一偶数，可分解为n1和n2两个数，
分别检查n1和n2是否为素数，如都是，则为一组解。
如n1不是素数，就不必再检查n2是否素数。先从n1=3开始，
检验n1和n2（n2=N-n1）是否素数。然后使n1+2
再检验n1、n2是否素数，… 直到n1=n/2为止。
我的代码：
 Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
        Label2.Text = ""
        Dim a, n, i, j, k, fir, sec As Integer, flag, fla As Boolean
        a = Val(TextBox1.Text)
        For n = 3 To a / 2 Step 2
            flag = True
            For i = 2 To (n - 1)
                If n Mod i = 0 Then
                    flag = False : Exit For
                End If
            Next i
            If flag Then
                fir = n
                sec = a - n
                For k = 3 To sec / 2 Step 2
                    fla = True
                    For j = 2 To (k - 1)
                        If n Mod j = 0 Then
                            fla = False : Exit For
                        End If
                    Next j
                    If fla Then
                        Label2.Text += a.ToString + "=" + fir.ToString + "+" + sec.ToString + Chr(10)
                    ElseIf fla = False Then
                        Exit For
                    End If
                Next k
            ElseIf flag = False Then
                Exit For
            End If
        Next n
    End Sub
End Class
结果出来的不对，一样的结果出来好几遍，求大神帮改！！！