Google Sheets Increment with INDEX

I have the formula:

=IF(INDEX(INDIRECT("Rota!"& $B$1):Rota!$Z$1000, 1, 2) = "", "-", INDEX(INDIRECT("Rota!"& $B$1):Rota!$Z$1000, 1, 2))

I would like to increment it by 1 every time I drag it down.
How can I do this?