Adding +1 to a text
 

I have a column where a user will input Item 1, Item 2, etc. etc. within a
range. These inputs are not necessarily contiguous but they will be ascending:

A1 Item 1
A2 Subtotal 1
A3 Item 2
A4
A5 Subtotal 2
A6 Item 3
Is there a function that when a user inputs "Item" the auto fill places a 1
or 2 if 1 has already been input, and 3 if Item 2 is already used?

Thanks for any light you can give me on this.
Regards,