Sum Across sheets

I have a series of 12 sheets each named as a month
I wish to find the cumulative value of a cell in each month

So the January sheet would have a value .Range("AP9").Value
the February Sheet would have a value January!AP9 + February!AP9
March January!Ap9 + February!AP9 + March!AP9 and so on

I triedto achieve this with the following code

Sub updatesummary()
Dim ws as workSheet
Sformula = ""
For Each ws In ActiveWorkbook.Worksheets

    With Sheets(ws.Name)
   
        If ws.Name <> "Template" Then
                 If ws.Name <> "Names" Then
                    If Left(ws.Name, 3) <> "SUM" Then
    
                            
                                 Sformula = Sformula & ws.Name & "!" & .Range("AP9").Address & " +"
                            
                                   Sformula = "= " & Left(Sformula, Len(Sformula) - 2)
                                   .Range("AQ9").FormulaR1C1 = Sformula
                                   Sformula = Sformula & "+"
                                   Sformula = Right(Sformula, Len(Sformula) - 3)
                            .Range("AQ9").Copy
                                .Range("AQ10:AQ57").PasteSpecial Paste:=xlPasteFormulas
                                
                                Range("AQ9:AQ57").Copy
                                Range("AQ9:AQ57").PasteSpecial Paste:=xlPasteValues
                                Application.CutCopyMode = False

                            
                     End If
             End If
        End If
       

 End With
Next

   
End Sub

But this does nor work, the initial = sign causes an error, but without the sign all I have is a string not a formula
I need some help please
LoftySuth

I was also working on this.

combining my effort with Jakobshavn's:-

Sub YearToDate()
    Sheets(1).Range("B10").Formula = "=AP9"
    For i = 2 To 12
        Sheets(i).Range("B10").Formula = "=" & Sheets(i - 1).Name & "!B10+AP9"
    Next i
End Sub

Alternatively you could use the Month Names instead of assuming that the sheets are sequential.

This is probably the most robust solution.

Sub YearToDate2()
    Sheets(MonthName(1)).Range("B10").Formula = "=AP9"
    For i = 2 To 12
        Sheets(MonthName(i)).Range("B10").Formula = "=" & Sheets(MonthName(i - 1)).Name & "!B10+AP9"
    Next i
End Sub

it occurs to me that you might want to sort the sheets first.

For ws = 1 To 11
Sheets(MonthName(ws)).Move before:=Sheets(ws + 1)
Next
End Sub

Thank you all for your ideas. The key revelation to me was Sheets.Range.Formula. I don't really want to use a cumulative Cell (B10) but I think I will be able to write an accumulating string as in my original post (SFormula = SFormula &.....). Once again many thanks for all your contributions my jaw dropped when I opened up the Forum!