Formula not staying

Hello community!  I am having trouble with my worksheet. It is not keeping my formula for adding number from different worksheets.  The formula that I am trying is: ='1'!C12+'2'!C12+'3'!C12+'4'!C12+'5'!C12+'6'!C12+'7'!C12+'8'!C12.  When I save, close and reopen the file, the formula disappears and the resulting number is kept.  Please advise on trouble shooting with this concern. Thanks!