calculate average and put the output in listbox

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VB

Visual basic problem, I need to calculate average and put the output in listbox but i'm having an error?
1. Write a program to compute semester averages. Each set of five lines in a text file should contain a student's social security number and the grades for three hourly exams and the final exam. (The final exam counts as two hourly exams.) the program should display each student's Social Security number and semester average, and then the class average. Use the data in Table : Students grades: http://i54.tinypic.com/2gya6j6.jpg[^]

and this is what i did:

Public Class Form1

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim sr As IO.StreamReader = IO.File.OpenText("AVERAGE.TXT")
Dim SSN, Exam1, exam2, exam3, finalexam, average As Double
SSN = CDbl(sr.ReadLine)
Exam1 = CDbl(sr.ReadLine)
exam2 = CDbl(sr.ReadLine)
exam3 = CDbl(sr.ReadLine)
finalexam = CDbl(sr.ReadLine)
average = CDbl((Exam1 + exam2 + exam3 + finalexam + finalexam) / 5)
Dim fmtstr As String = "{0,-13} {1,6}"
With ListBox1.Items
.Clear()
.Add(String.Format(fmtstr, "Soc. Sec. No.", "Average"))
.Add(String.Format(fmtstr, SSN, average))
.Add(String.Format(fmtstr, SSN, average))
.Add(String.Format(fmtstr, SSN, average))
End With
End Sub
End Class

but every time i click on the button it's ending the program :(
please i need your help