$10 - 1 = 9$ 
Brown wants to put some board between his books. He has $10$ books in total. If he wants to put one board between every two adjacent books, how many boards does he need to prepare? 
Options:
1: $$9$$ 
2: $$10$$ 
3: $$11$$ 
4: $$12$$ 
5: $$13$$ 