Matlab - sum of diagonals of a matrix

Hello

i need to replicate the "matrix" proccess type of a discrete function convolution. As shown in this video: https://www.youtube.com/watch?v=N6Ih6lV6Ses
i already created a code that makes the matrix, but now i have to do the respective sums in the diagonal way, but i cannot get with the best way to do it.

Help with any ideas are apreciated

Source: https://es.mathworks.com/help/matlab/ref/double.trace.html