The frequency distribution of the compressive strength of $20$ concrete cube specimen is given in the table.
$$\begin{array}{|c|c|} \hline f \text{ (MPa)} & \text{Number of specimens with compressive strength equal to }f \\ \hline 23 & 4 \\ \hline 28 & 2 \\ \hline 22.5 & 5 \\ \hline 31 & 5 \\ \hline 29 & 4 \\ \hline \end{array}$$
If $\mu$ is the mean strength of the specimens and $\sigma$ is the standard deviation, the number of specimens (out of $20$) with compressive strength less than $\mu - 3 \sigma$ is ____