Consider the following four processes with arrival times (in milliseconds) and their lengths of CPU bursts (in milliseconds) as shown below :

Process	P1	P2	P3	P4
Arrival time	0	1	3	4
CPU burst time	3	1	3	Z

These processes are run on a single processor using preemptive Shortest Remaining Time First scheduling algorithm. If the average waiting time of the processes is 1 millisecond, then the value of Z is