|Professor Smith teaches a logic class. One day, he writes some statements on the blackboard:
Exactly a of these statements are true.
Exactly b of these statements are true.
Exactly c of these statements are true.
Each of a, b, c and so on is a number. He then asks the class how many of the statements are true.
You will be given a int statements, containing the numbers written in Professor Smith's statements. Return the number of the statements that are true. If there is more than one possible answer, return the largest one. If there are no possible answers, return -1.