Vision Problem using symbolic MATLAB toolbox

I am trying to solve a vision problem in robotis deals with epipolar transforms using symbolic tool box of MATLAB. Tried this but get wrong answer.

F = str2sym('F(x, y)');
G = str2sym('G(x, y)');

% write down the formula to compare two image patches via SSD 
for x=-2:2
    for y=-2:2    
e=symsum(symsum((F-G).^2));
    end
end