What is the missing number?

A preacher, newly called to a small country town, needed to mail a letter. Passing a young boy on the street, the pastor asked where he could find the post office. After getting his answer, the minister thanked the boy and said, “If you’ll come to the community church this evening, you can hear me tell everyone how to get to heaven.” “I don’t know, sir,” the boy replied. “You don’t even know how to get to the post office!”