Разработать программу обчисления суммы факториалов всех целых чисел от 1 до N

Option Explicit
'На форме кнопка Command1, Text1 для ввода числа и Text2 (побольше размером и с установленным MultiLine=True) для вывода результата
Private Sub Command1_Click()
Dim i As Long, fa As String, s As String
fa = 1
s = 0
For i = 1 To Val(Text1.Text)
  fa = mul(fa, CStr(i))
  s = add(s, fa)
Next i
Text2.Text = s
End Sub
 
'сложение
Public Function add(m1 As String, m2 As String) As String
Dim i As Long, L1 As Long, L2 As Long
Dim a() As Byte, b1() As Byte, b2() As Byte
L1 = Len(m1)
L2 = Len(m2)
If L1 > L2 Then
  b1 = "0" & m1
  b2 = String$(L1 - L2 + 1, "0") & m2
  a = String$(L1 + 1, "0")
Else
  b2 = "0" & m2
  b1 = String$(L2 - L1 + 1, "0") & m1
  a = String$(L2 + 1, "0")
End If
For i = UBound(a) - 1 To 2 Step -2
  a(i) = a(i) + b1(i) + b2(i) - 96
  If a(i) > 57 Then
    a(i) = a(i) - 10
    a(i - 2) = 49
  End If
Next i
If a(0) = 48 Then a(0) = 32
add = Trim$(a)
End Function
 
'вычитание
Function dif(m1 As String, m2 As String) As String
Dim a() As String, n As Byte
Dim i1 As Long, i2 As Long, L1 As Long, L2 As Long
  L1 = Len(m1): L2 = Len(m2)
  ReDim a(L1)
  For i1 = L1 To 1 Step -1
    If L2 > 0 Then
      a(i1) = Mid$(m1, i1, 1) - Mid$(m2, L2, 1) - i2
      L2 = L2 - 1
    Else
      a(i1) = Mid$(m1, i1, 1) - i2
    End If
    If a(i1) < 0 Then
      a(i1) = a(i1) + 10
      i2 = 1
    Else
      i2 = 0
    End If
  Next i1
  For i1 = 1 To L1 - 1
    If a(i1) = "0" Then a(i1) = "" Else Exit For
  Next i1
  dif = Join(a, "")
End Function
 
'умножение
Function mul(m1 As String, m2 As String) As String
Dim a() As String, n As Byte
Dim i1 As Long, i2 As Long, L1 As Long, L2 As Long
  L1 = Len(m1): L2 = Len(m2)
  ReDim a(L1 + L2)
  For i2 = L2 To 1 Step -1
    n = Mid$(m2, i2, 1)
    For i1 = L1 To 1 Step -1
      a(i1 + i2) = Val(a(i1 + i2)) + n * Mid$(m1, i1, 1)
      If a(i1 + i2) > 9 Then
        a(i1 + i2 - 1) = Val(a(i1 + i2 - 1)) + a(i1 + i2) \ 10
        a(i1 + i2) = a(i1 + i2) Mod 10
      End If
    Next i1
  Next i2
  mul = Join(a, "")
End Function
 
'деление
Function div(m1 As String, m2 As String) As String
Dim dm As String
Dim i1 As Long, i2 As Long, L1 As Long, L2 As Long
  L1 = Len(m1): L2 = Len(m2)
  dm = Mid$(m1, 1, L2)
  For i1 = 1 To L1 - L2 + 1
    i2 = 0
    While StrCompEx(dm, m2) >= 0
      dm = dif(dm, m2)
      i2 = i2 + 1
    Wend
    If dm = "0" Then dm = ""
    dm = dm & Mid$(m1, L2 + i1, 1)
    div = div & IIf(Len(div) = 0 And i2 = 0, "", i2)
  Next i1
End Function
 
'Сравнение 2-х положительных целых сверхбольших чисел в строковом формате
Function StrCompEx(m1 As String, m2 As String) As Integer
  StrCompEx = Sgn(Len(m1) - Len(m2))
  If StrCompEx = 0 Then StrCompEx = StrComp(m1, m2)
End Function