Adding The Same Cell In All Sheets

I have found following example:

This month I will show you some handy formulas used with a twist. SUM ALL SHEETS.
The first one is how we can use the SUM function to Sum, say cell A1, on all Worksheets in your Workbook. With this method, all new sheets that are added to the Workbook are included in the SUM. Here is how;

Add a new Worksheet to you Workbook at the far right. To do this select "Worksheet" under "Insert" on the Worksheet Menu Bar. Then left click on this new Worksheet name tab, holding down the the left mouse button drag to the far right and release.
Name this sheet (double click the name tab) "Spacer" (although any name will do).
With "Spacer" being the active sheet, go to Format>Sheet>Hide. Repeat steps 1 to 3 above, but this time name the Worksheet "Start " and drag it to the far left. On the Worksheet you wish to have the result shown on enter: =SUM('Start :Spacer'!A1)................