求助如何跨工作薄求和

Option Explicit
Sub abc()
　Dim pth, filename(), i, wb, sum
　pth = "c:\abc\12月工资" '指定目录，自己修改
　If Dir(pth, vbDirectory) = vbNullString Then MsgBox pth: Exit Sub
　If Not getfilename(filename, pth, ".xlsx") Then MsgBox "!": Exit Sub
　Application.ScreenUpdating = False
　For i = 1 To UBound(filename)
　　Set wb = Workbooks.Open(filename(i))
　　With wb
　　　With .ActiveSheet '当前工作表，可以指定
　　　　sum = sum + .[G100].Value
　　　End With
　　　.Close False
　　End With
　Next
　[a1] = sum '写入当前工作表[a1]单元格
　Application.ScreenUpdating = True
End Sub
Function getfilename(filename, pth, mark) As Boolean
　Dim f, n
　If right(pth, 1) <> "\" Then pth = pth & "\"
　f = Dir(pth & "*.*")
　Do While Len(f) > 0
　　If LCase(right(f, Len(mark))) = LCase(mark) Then
　　　If left(f, 1) <> "~" Then
　　　　n = n + 1
　　　　ReDim Preserve filename(1 To n)
　　　　filename(n) = pth & f
　　　End If
　　End If
　　f = Dir
　Loop
　If n > 0 Then getfilename = True
End Function

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