Find the summation of the outer elements of a rectangular matrix

Using this matrix

 a=[2 -3 1 0;5 7 1 -4;9 -6 0 2]

I need to find the sum of all the outer numbers in this matrix using loops and/or conditional statements. I have absolutely no idea on how to go about starting/doing this. Any help is appreciated, thank you.

Answer 1

[Andrei Bobrov]

a=[2 -3 1 0;5 7 1 -4;9 -6 0 2]

t = true(size(a))
t(2:end-1,2:end-1) = false
out = sum(a(t))

Answer 2

[Azzi Abdelmalek]

   out=sum([a(1:end-1,1)' a(end,1:end-1) a(2:end,end)' a(1,2:end)])