How do I Implement the Gaussian Kernel in Matlab to improve contrast

I want to implement this equation using matlab I have done the exponential part using these codes but, I can't figure out how to get K

[m,n]=size(I)
c=70;
for x=1:m
    for y=1:n
        D(x,y)= exp(-(x^2+y^2)/c^2);
    end
end

if any one could help, I would really appreciate it