Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Model | Availability | Introduced | Length of use | Last of whiteness |
| --- | --- | --- | --- | --- |
| Crest Whitestrips Classic\npreviously Crest Whitestrips | Discontinued | May 2001 | 14 days | 12 months |
| Crest Whitestrips Professional | Discontinued | 2001 | 10 days | 12 months |
| Crest Whitestrips Supreme | Dentist and online | September 2003 | 21 days | 18 months |
| Crest Whitestrips Premium | Discontinued | January 2004 | 7 days | 12 months |
| Crest Whitestrips Pro\npreviously Crest Whitestrips Premium Plus | Discontinued | April 2005 | 10 days | 18 months |
| Crest Whitestrips Renewal | Discontinued | January 2006 | 10 days | 18 months |
| Crest Whitestrips Daily Multicare | Discontinued | March 2007 | Daily | White after using system |
| Crest Whitestrips Advanced Seal | Discontinued | February 2009 | 14 days | 18 months |
| Crest Whitestrips 3D Vivid | Instore and online | 2009 | 10 days | 12 months |
| Crest Whitestrips 3D Advanced Vivid | Instore and online | March 2010 | 14 days | 12 months |
| Crest Whitestrips 3D Professional Effects | Instore and online | March 2010 | 20 days | 12 months |
| Crest 3D White 2 Hour Express | Instore and online | 2010 | 2 hours | 3 months |
| Crest 3D Intensive Professional Effects | Instore and online | 2011 | 7 days | 12 months |
Question:
how many models require less than a week of use?
Answer:
2