Need a formula for tracking a number

[SirCumferance]

So, I have a number in B1, 1-21. I need a formula in A1 that will return a value based on that number.
i have a method that looks goodish, but not sure if efficient and/or wrong, even if it looks like the values are good.
I want it so that at 0,1,2,3,4,5,6,7,8,9 it returns 0,0,-1,-1,-1,-2,-2,-2,-3, etc. But I seem to be missing something, most likely simple. I am toying with

=-1*CEILING(M12/2,1)

to varying degrees of success. I can get the smaller numbers to work, but not the higher ones, lol.

Anyone can help this math deficient individual?

 

[Joe4]

Try this:

=INT((1-M12)/3)

 

[SirCumferance]

SO CLOSE!
It starts the -1 at 2, instead of 3, so the whole thing is a full number off.
BUT, you have given me another method to play with, thank you.

 

[SirCumferance]

DUDE!
=INT((2-I970)/3) and so far, it is working.
Many thanks

 

[Joe4]

Glad you got is working.

Regarding your previous post:

It starts the -1 at 2, instead of 3, so the whole thing is a full number off.

that was on purpose, per your original request:

I want it so that at 0,1,2,3,4,5,6,7,8,9 it returns 0,0,-1,-1,-1,-2,-2,-2,-3, etc.

So, if you go line-by-line:
0=0
1=0
2=-1
3=-1
etc.

In hindsight, I am guessing that was a typo in the original request.

 

[SirCumferance]

Ah man, yep, typo... 0,0,-1,-1,-2, etc
Thanks man, very helpful