Consider the following processes, with the arrival time and the length of the CPU burst given in milliseconds. The scheduling algorithm used is preemptive shortest remaining-time first.$$\small \begin{array}{|c|c|c|} \hline \textbf{Process} & \textbf{Arrival Time} & \textbf{Burst Time}\\\hline \text{$P_1$} & 0 & 10 \\\hline \text{$P_2$} & 3 & 6 \\\hline \text{$P_3$} & 7 & 1 \\\hline \text{$P_4$} & 8 & 3 \\\hline \end{array}$$The average turn around time of these processes is ___________ milliseconds.