MATHS-coutulating the squares on the edge of an even grid

Hello. In math today Mr Ress got us to work out how many squares are in the outside of a 10 x 10 square. We have found 6 ways of doing said ecation. two of the ways is on that I found which is  (n-1)+(n-1)+(n-1)+(n-1)= a
A= the answer and N= the number so this turns into... (10-1)+(10-1)+(10-1)+(10-1)=36 or it could be 9+9+9+9=36. Are you confused yet? Because I am. If we were to simplify this then it would be 4(n-1)=a or 4 x 9 = 36. Now if you are wondering how I got n-1,  it is because when you count the sides you end up counting the corners twice and so you just take one  AGGG!  well I can't explain. Um here is a picture that explains it a bit better than I can

Another way, is 4n -4. Or 4 x 10 - 4 because you have your 10x4=40 but you have counted your corners twice so you just get rid of one of the times by taking away 4 so you have your anwer, 36.

here are all the ways to do it 

I know this was hard to understand, I can't seem to find a way to explain this easily. Sorry for making it so hard to understand. I wouldn't of posted this but I had to. Sorry. Can you think of any other ways of doing this?