Following data refers to the jobs $(P, Q, R, S)$ which have arrived at a machine for scheduling. The shortest possible average flow time is __________ days.
$$\begin{array}{|l|l|} \hline \text{Job} & \text{Processing Time (days)} \\ \hline P & 15 \\ \hline Q & 9 \\ \hline R & 22 \\ \hline S & 12 \\ \hline \end{array}$$