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CCC 2013 - Problem J1: Next In Line

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 know a family with three children. Their ages form an arithmetic sequence: the difference in ages between the middle child and youngest child is the same as the difference in ages between the oldest child and the middle child. For example, their ages could be 5, 10 and 15, since both adjacent pairs have a difference of 5 years.

Given the ages of the youngest and middle children, what is the age of the oldest child?

Input Specification
The input consists of two integers, each on a separate line. The first line is the age Y of the youngest child (0 ≤ Y ≤ 50). The second line is the age M of the middle child (Y ≤ M ≤ 50).

Output Specification
The output will be the age of the oldest child.

Sample Input 1
12
15

Output for Sample Input 1
18

Sample Input 2
10
10

Output for Sample Input 2
10

My solution (in VB):

Option Strict On
Public Class Form1

    Private Sub btnDisplay_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnDisplay.Click

        Dim Y As Integer
        Dim M As Integer

        Try
            Y = CInt(txtYoungest.Text)
            M = CInt(txtMiddle.Text)

            If Y > M Or M > 50 Then
                MsgBox("Please make sure the middle age is greater than or equal to the youngest age and their ages are no greater than 50.")
                txtYoungest.Text = ""
                txtMiddle.Text = ""
            Else
                Dim d As Integer, O As Integer

                d = M - Y

                O = M + d

                txtOldest.Text = CStr(O)

            End If
        Catch ex As Exception
            MsgBox("Please enter integer numbers.")
            txtYoungest.Text = ""
            txtMiddle.Text = ""
        End Try
    End Sub

    Private Sub txtYoungest_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles txtYoungest.TextChanged
        If txtYoungest.Text.StartsWith(".") Then
            txtYoungest.Text = ""
            MsgBox("Please enter integer numbers.")
        End If
    End Sub

    Private Sub txtMiddle_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles txtMiddle.TextChanged
        If txtMiddle.Text.StartsWith(".") Then
            txtMiddle.Text = ""
            MsgBox("Please enter integer numbers.")
        End If
    End Sub
End Class

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