Use the divergence theorem to evaluate $\iint_{S} \mathbf{F} \cdot d \mathbf{S},$ where $\mathbf{F}(x, y, z)=x y \mathbf{i}-\frac{1}{2} y^{2} \mathbf{j}+z \mathbf{k}$ and $S$ is the surface consisting of three pieces: $z=4-3 x^{2}-3 y^{2}, 1 \leq z \leq 4$ on the top; $x^{2}+y^{2}=1,0 \leq z \leq 1$ on the sides; and $z=0$ on the bottom.