A large conference has 500 attendees. Each attendee receives a gift bag that contains 3 pens, 2 notebooks, and 1 water bottle. The conference organizers ordered 1,800 pens, 1,000 notebooks, and 600 water bottles. How many more notebooks are needed to ensure each attendee receives the full gift bag? Let's think step by step and output the final answer within \boxed{}.
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