Global Indexing in For loops

Dear Community!

I have been working on a project where I find myself a little bit stuck:

I have elements, where each of them has 4 3x3 matrices associated. In total I have 215 Elements and therefore, I have a cell array of 430x2 dimension. The next step is to obtain a 215x215 diagonal matrix according to the picture attached. The problem I am currently facing is that I need to perform the following sum :

the sum of the 4th matrix of the (n-1)th element + the 1st matrix of the nth element. I know I need some sort of global index or something to tell the script how to perform the addition but I am not familiar with the syntax. Do you have any answer to that?

Thanks!

for i = 1:2:215
    for j = 1:2:215
        
        if (i==1) && (i==j)
            
            Structure_Global_Stiffness{i,i} = Resultant_K{i,1};
            Structure_Global_Stiffness{i,i+1} = Resultant_K{i,2};
        elseif (i==j) && (i<215)
            Structure_Global_Stiffness{i,i-1} = Resultant_K{i,1};
            Structure_Global_Stiffness{i,i} = Resultant_K{i,2} + Resultant_K{i+1,1};
            Structure_Global_Stiffness{i,i+1} = Resultant_K{i+1,2};
        elseif (i==j) && (i==215)
            Structure_Global_Stiffness{i,i-1} = Resultant_K{i,1};
            Structure_Global_Stiffness{i,i} = Resultant_K{i,2};
        elseif (j < i-1)
            Structure_Global_Stiffness{i,j} = zeros(3); 
        elseif (i==1) && (j > i+1)
            Structure_Global_Stiffness{i,j} = zeros(3);
        elseif (i > 1) && (j > i+1)
            Structure_Global_Stiffness{i,j} = zeros(3);
        end
        
    end
end