how to find stair phase ?

i have stair phase equation and i am trying to generate but its not working properly, so please help my code is: clc; clear all; m=256; n=256; p=100; N=4; for i= 1:m for j=1:n SP(i,j)=-pi+round(mod(i,p))*(2*pi/N); end

end plot(SP);

please find attachment