How to auto number merged cells in Excel?

How could we fill series numbers into a list of different size merged cells in Excel? First in our mind is to drag the Autofill handle to fill the merged cells, but, in this case, we will get the following warning message, and can’t fill the merged cells.
doc-fill-merged-cells-1
Since the fill handle does not support the merged cells, here, I can talk about some other tricks for solving this problem.
Auto number / fill merged cells with VBA code

arrow blue right bubble Auto number / fill merged cells with VBA code

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The following VBA code can help you to number the selected merged cells quickly. Please do as follows:
1. Hold down the ALT + F11 keys, and it opens the Microsoft Visual Basic for Applications window.
2. Click Insert > Module, and paste the following code in the Module Window.
VBA code: Auto number merged cells
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Sub NumberCellsAndMergedCells()
    'Update 20141028
    Dim Rng As Range
    Dim WorkRng As Range
    On Error Resume Next
    xTitleId = "KutoolsforExcel"
    Set WorkRng = Application.Selection
    Set WorkRng = Application.InputBox("Range", xTitleId, WorkRng.Address, Type: = 8)
    Set WorkRng = WorkRng.Columns(1)
    xIndex = 1
    Set Rng = WorkRng.Range("A1")
    Do While Not Intersect(Rng, WorkRng) Is Nothing
        Rng.Value = xIndex
        xIndex = xIndex + 1
        Set Rng = Rng.MergeArea.Offset(1)
    Loop
End Sub
3. Then press F5 key to run this code, and a prompt box will pop out to let you select the merged cells you want to fill, see screenshot:
doc-fill-merged-cells-1
4. After selecting the merged cells, and click OK, now, your selected merged cells have been filled with sequential numbers, see screenshot:
doc-fill-merged-cells-1