A project consists of eight activities. The time required for each activity and its immediate predecessor(s) are given in the table below.

Activity	Activity time (in days)	Immediate predecessor(s)
A	2	-
B	3	-
C	2	A
D	4	A, B
E	4	C
F	3	C
G	X	D, E
H	2	F, G

If the project completion time using critical path method (CPM) is 15 days, then the value of X (in days) is ________. [in integer]