MATLAB: How to Sum matrix diagonals

Question

If I have the following matrix

a = [1 2 3; 4 5 6; 7 8 10
a =
     1     2     3
     4     5     6
     7     8    10

How do I sum the diagonal of 7,5,3? I know that to sum 1,5,10 I use

sum(diag(a))

But when I try

sum(diag(a,2))

The answer is 3. Is there a built in function that calculates the sum, or do I have to make a while loop?

Answer 1

sum(diag(a(:,end:-1:1)));

or summing all diagonals:

sum(spdiags(rot90(a)));