function x = Gauss(A, d)
n = length(d);
for i = 1 : n % each column
  if A(i, i) == 0 then
    error('zero in the first column.');
  end
  for j = i+1 : n % each row
    k = - A(j, i) / A(i, i);
    A(j, :) = A(j, :) + A(i, :) * k;
    d(j, 1) = d(j, 1) + d(i, 1) * k;
  end
end

x = zeros(n, 1);
x(n) = d(n, 1) / A(n, n);
for i = n-1 : -1 : 1
  x(i) = (d(i, 1) - A(i, :) * x) / A(i, i);
end