add return value to the main function.
	modified:   ex1-10.c
	modified:   ex1-8.c
	modified:   ex1-9.c
	modified:   sec1-3b.c
	modified:   sec1-4.c
	modified:   sec1-5a.c
	modified:   sec1-5b.c
	modified:   sec1-5c.c
	modified:   sec1-5d.c