Hi
Try
=SUM(Sheet1:Sheet20!A1)
Change the value of sheet20 to the highest sheet number in your book.
Be aware however, that if the sheets are not in sequence, you may not
pick up the correct answer.
I prefer to create 2 new sheets called First and Last and drag these to
encompass the range of sheets I am interested in.
The
=SUM(First:Last!A1)
will total all A1 cells within the "sandwich".