Auto fill doesn't work on this sequence

[rifpool]

I've got a sheet that goes...

1
1
2
2
3
3

I want to highlight those and drag it down so it looks like...

1
1
2
2
3
3
4
4

But at 4 I get this

3.6
4.057142857

Any ideas on how to do this? MS Office Plus 16.5

 

[Marcelo Branco]

Say the numbers are in A1:A6

A7
=INT(COUNT(A$1:A6)/2)+1
copy down

M.