How many piers does a harbor master need for accommodating six cruise

How many piers does a harbor master need for accommodating six cruise ships S1, · · ·, S6 with expected dates of arrival A and departure D in July, (A, D) = (10, 13), (13, 15), (16, 18), (14, 17), (12, 15), (16, 18), (14, 17), respectively, if each pier can accommodate only one ship, arrival being at 6 am and departures at 11 pm? Join Si and Sj by an edge if their intervals overlap. Then color vertices.

PROBLEM SET 23.3: