At Halloween Mr. Goodfellow decides to give 2 items to each trick or treater. He starts with 31 candy bars, 22 apples, and 21 packs of gum. He first gives 1 candy bar and 1 apple, then 1 candy bar and 1 pack of gum and continues in this order. At the end of the night he has the same number of all three items. How many trick or treaters came to his house?