Increase function formula by 1 each time i drag down

I am trying to do this:

=Unique(Input!$C$4:$C$20000,9)
=Unique(Input!$C$4:$C$20000,10)
=Unique(Input!$C$4:$C$20000,11)
=Unique(Input!$C$4:$C$20000,12)


but I have to manually type 9, 10, 11, 12 and its taking forever because I have to do it up to 100. Is there a better way?