KöMaL 2006 K97

There are 7 houses in Dragon Street, numbered 1 to 7. Last week, Mr. D. E. Livery the postman delivered letters to two houses every day. There are two houses where he did not brought anything during the whole week, but he visited all the other houses twice. The sum of the number of the houses visited by Mr. Livery was 5 on Monday, 8 on Tuesday, 9 on Wednesday, 13 on Thursday and 7 on Friday. (There is no mail on Saturday and Sunday.) Which are the two houses that did not receive any letter last week?