How to include a second loop in my script

I have created the following script and it works ok. However, I need to somehow include a second loop so that every Y matrix is calculated upon the previous one and not on the initial CLN matrix. For example, the first step gives Y = 56 for the first cell, so I want the second step to calculate Y = 56+0.5*B(i,j) and so on. Any suggestions? Thank you :)

S = [12 4 6 4;16 12 2 3;0 4 10 12;4 12 21 3];
CLN = 50*ones(4);
for t = 0:4
    B = [S - 2*t];
    B(B<0)=0
    Y = zeros(size(B));
    for i = 1:4
        for j = 1:4
            if (B(i,j)<5);
                Y = CLN(i,j) - 0.5*B(i,j)
            elseif(CLN(i,j)-0.5*B(i,j)<0)
                Y = 0
            else(B(i,j)>=5);
                Y = CLN(i,j) + 0.5*B(i,j)
            end
        end
    end
end

[Rik]

This time I edited your question for you. Next time, please use the tools explained on this page to make your question more readable.