AutoFill Problem with single letter and numbers, without using formula or custom list
 

I'm trying to autofill the following and it keeps repeating, without using a formula, only formatting, can someone help please?
when I put in: a20170000001, a20170000002, a20170000003, a20170000004, it only repeats.
I can't use a custom list, as the fields will never repeat, only grow.
the only items changing will be the numeric value at the end, going up to possibly 90,000 (ie; a20170090000)