Pointing to the Poynting!

Find the mean Poynting vector of a plane electromagnetic wave $\textbf{E} = {\textbf{E}}_{m}cos(\omega t - \textbf{k}\textbf{r})$ if the wave propagates in vacuum.

If the answer can be written as

$\displaystyle {S}_{av} = \frac{a}{b} {{\mu}_{0}}^{c}{{\textbf{E}}_{m}}^{d}{\textbf{k}}^{e}{\omega}^{f}$

where $a,b,c,d,e$ are positive reals.

Give your answer as $\displaystyle a + b + c + d + e + f$