problem with program

Question

___ could anyone tell me why i get an error on the line: letters = input.Substring((count), 2) it keeps saying the index is not with the substring but i don't see why that is _____ Function TotalLetterCount(ByVal input As String) 'input string is processed character by character 'and keeps track of how many of each character is found Dim count As Integer 'position in string Dim length As Integer 'length of input string Dim letters(25) As Integer 'array letters Dim arrayElement As Integer 'arrayElement of unicode value Dim letterCount As Integer 'integer to be used for 'total number of letters in the string length = input.Length count = 0 'processes character by character and keeps track of 'how many of each letter are found and updates the 'letters array For count = 0 To (length - 1) If (unicodeFor(input.Substring((count), 1)) >= 65) And _ (unicodeFor(input.Substring((count), 1)) <= 90) Then arrayElement = ((unicodeFor(input.Substring((count), 1))) - 65) letters(arrayElement) = 1 + letters(arrayElement) End If Next count 'adds items to letters listbox for all letters found 'and updates lettercount variable For count = 0 To 25 If (letters(count) >= 1) Then lstLetters.Items.Add((ChrW(count + 65)) & ": " & _ letters(count)) letterCount += letters(count) End If Next count 'places letterCount integer into TotalLetterCount 'textbox lblTotalLetterCount.Text = letterCount End Function Function DigramCount(ByVal input As String) Dim count As Integer 'count is position in string Dim length As Integer 'length of input string Dim nextLetter As Integer Dim letter1 As Char Dim letter2 As Char Dim digram(58, 58) As Integer Dim row As Integer Dim column As Integer Dim rowIndex As Integer Dim columnIndex As Integer Dim letters As String 'adds spaces to end and beginning of input string input = " " & input & " " length = input.Length count = 0 For count = 0 To (length - 1) letters = input.Substring((count), 2) letter1 = CChar(letters.Substring(0, 1)) letter2 = CChar(letters.Substring(1, 1)) arrangeLetters(letter1, letter2) row = ((AscW(letter1)) - 32) column = ((AscW(letter2)) - 32) If ((row >= 0) And (row <= 58)) Then If ((column >= 0) And (column <= 58)) Then digram(row, column) = 1 + digram(row, column) End If End If Next count For row = 0 To 58 For column = 0 To 58 If digram(row, column) >= (1) Then lstDigrams.Items.Add((ChrW(row + 32)) & _ (ChrW(column + 32)) & ": " & digram(rowIndex, columnIndex)) End If Next column Next row End Function