How to specify enties of a block diagonal matrix

c = input('c ');

g = input('g ');

M = cell(c+1,1);

a_t = min([1:c+1;repmat(c-g,1,c+1)]);

for i = 1:c+1

MCell = repmat({sym([char(i-1+'A') '%d%d'], [c+2-i c+2-i])}, 1, a_t(i));

M{i} = blkdiag(MCell{:});

end

B=blkdiag(M{:});

I am trying to make diagona matrix such that the block matrices from 1 to (c-g) are zero matrix and block matrix from (c-g+1) to c are only diagonal matrix with only first entry which is 'a'. Can someone please help me here?