Calculate $\iint_{\mathcal{S}} f(x,y,z)\,dS$ For $$x^2+y^2=4 \text{,} \quad 0\le z\le 6 \text{;}\qquad f(x,y,z)=e^{-z}$$ $\iint_{\mathcal{S}} f(x,y,z)\,dS=$