Computing Delta for mfcc

I am trying to make a code for to compute delta for mfcc but I am confused to what to do with Ct-n as it will show me an error everytime

from what I understand I the answer's dimensions should be similar to the mfcc static for my case it should be 19. but I am confused to how to compute it.

This is the code I made

Nn = 4;
dt = 0;
ct = 0;
for nN = 1:Nn
    dt(nN) = sum((nN*(ct(bandfftmag_static + nN)+ct(bandfftmag_static - nN))))/(2*sum(nN^2));
end