CCC 2013 - Problem J3: From 1987 to 2013

I actually wrote this contest and this is the code I used to be marked. I've since fixed up the code and rewrote it in Java. Here is a link to that fixed up code.

The problem:

You might be surprised to know that 2013 is the first year since 1987 with distinct digits. The years 2014, 2015, 2016, 2017, 2018, 2019 each have distinct digits. 2012 does not have distinct digits, since the digit 2 is repeated.

Given a year, what is the next year with distinct digits?

Input Specification
The input consists of one integer Y (0 ≤ Y ≤ 10000), representing the starting year

Output Specification
The output will be the single integer D, which is the next year after Y with distinct digits.

Sample Input 1
1987

Output for Sample Input 1
2013

Sample Input 2
999

Output for Sample Input 2
1023

My solution (in VB):

Option Strict On
Public Class Form1

    Private Sub button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles button1.Click
        Try
            Dim y As Integer = CInt(txtY.Text)
            Dim l As String = CStr(txtY.Text)
            Dim num As Boolean = False
            Dim a As String, b As String, c As String, g As String, f As String

            If y <= 10000 And y > 0 Then

                Do Until num = True
                    y += 1
                    l = CStr(y)
                    If l.Length = 1 Then
                        num = True
                    End If

                    If l.Length = 2 Then
                        a = l.Substring(0)
                        a = a.Remove(1)
                        b = l.Substring(1)
                        If Not (a = b) Then
                            num = True
                        End If
                    End If

                    If l.Length = 3 Then
                        a = l.Substring(0)
                        a = a.Remove(1)
                        b = l.Substring(1)
                        b = b.Remove(1)
                        c = l.Substring(2)
                        If a = b Or a = c Or b = c Then
                        Else
                            num = True
                        End If
                    End If

                    If l.Length = 4 Then
                        a = l.Substring(0)
                        a = a.Remove(1)
                        b = l.Substring(1)
                        b = b.Remove(1)
                        c = l.Substring(2)
                        c = c.Remove(1)
                        f = l.Substring(3)
                        If a = b Or a = c Or a = f Or b = c Or b = f Or c = f Then
                        Else
                            num = True
                        End If
                    End If

                    If l.Length = 5 Then
                        a = l.Substring(0)
                        a = a.Remove(1)
                        b = l.Substring(1)
                        b = b.Remove(1)
                        c = l.Substring(2)
                        c = c.Remove(1)
                        f = l.Substring(3)
                        f = f.Remove(1)
                        g = l.Substring(4)

                        If a = b Or a = c Or a = f Or a = g Or b = c Or b = f Or b = g Or c = f Or c = g Or f = g Then
                        Else
                            num = True
                        End If

                    End If

                    If num = True Then
                        Dim D As Integer = y
                        txtD.Text = CStr(D)

                    End If

                Loop

            Else
                MsgBox("Please enter a positive integer for the year that is between 0 and 10 000")
                txtY.Text = ""
                txtD.Text = ""
            End If

        Catch ex As Exception
            MsgBox("Please enter an integer number for the year.")
            txtY.Text = ""
        End Try

    End Sub
End Class

'a = l.Substring(0)
'a = l.Remove(1)
'b = l.Substring(1)

'txtD.Text = a + Space(3) + b

'a = l.Substring(0)
'a = l.Remove(1)
'b = l.Substring(1)
'b = b.Remove(1)

'txtD.Text = a.PadRight(3) + b

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Test cases (as .in files):

Using their test cases:

All the following outputs are exactly the expected outputs.

Input:
3030
Output:
3041
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Input:
9029
Output:
9031
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Input:
1812
Output:
1820
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Input:
1867
Output:
1869
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Input:
1776
Output:
1780
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Input:
1000
Output:
1023
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Input:
2987
Output:
3012
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Input:
9
Output:
10
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Input:
10
Output:
12
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Input:
65
Output:
67
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Input:
279
Output:
280
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Input:
10000
Output:
10234
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Input:
0
Output:
1
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Input:
9876
Output:
10234
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Input:
987
Output:
1023
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