Students A, B, and C go to buy books together. The number of books they buy is two digits, and A buys the most books, and C buys the least. It is also known that the sum of these books is an even number. Their product is 3960, so B has the most How many books to buy?
甲乙丙三位同学一起去买书，他们买书的本数都是两位数字，且甲买的书最多，丙买的书最少，又知这些书的总和是偶数，它们的积是3960，那么乙最多买多少本书？
<result>18</result>