How to add a matrix?? with sym

n=input('How many nodes are there?')

C = sym(zeros(4,4));

D = sym(zeros(n,n));

% x_index = [1 2 5 6];

% y_index = [1 2 5 6];

x_nodes = [3 4 5 6];

y_nodes = [3 4 5 6];

for i = 1:size(C)

for j = 1:size(C)

D(x_nodes(i),y_nodes(j)) = k1(i,j);

end

end

D

C = sym(zeros(4,4));

D = sym(zeros(n,n));

% x_index = [1 2 5 6];

% y_index = [1 2 5 6];

x_nodes = [3 4 5 6];

y_nodes = [3 4 5 6];

for i = 1:size(C)

for j = 1:size(C)

J(x_index(i),y_nodes(j)) = k2(i,j);

end

end

J

C = sym(zeros(4,4));

U = sym(zeros(n,n));

% x_index = [1 2 5 6];

% y_index = [1 2 5 6];

x_nodes = [3 4 5 6];

y_nodes = [3 4 5 6];

for i = 1:size(C)

for j = 1:size(C)

U(x_index(i),y_nodes(j)) = k3(i,j);

end

end

U

sym(Kg)=D+J+U

I cannot get the Kg value..

I dont know why?