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:
| Overall Pick # | AFL Team | Player | Position | College |
| --- | --- | --- | --- | --- |
| 4.0 | Miami Dolphins | Bob Griese | Quarterback | Purdue |
| 5.0 | Houston Oilers | George Webster 1 | Linebacker | Michigan State |
| 6.0 | Denver Broncos | Floyd Little | Running Back | Syracuse |
| 12.0 | New York Jets | Paul Seiler | Offensive Guard | Notre Dame |
| 14.0 | San Diego Chargers | Ron Billingsley | Defensive Tackle | Wyoming |
| 17.0 | Oakland Raiders | Gene Upshaw | Offensive Guard | Texas A&I |
| 21.0 | Boston Patriots | John Charles | Defensive Back | Purdue |
| 22.0 | Buffalo Bills | George Daney | Offensive Guard | Arizona State |
| 23.0 | Houston Oilers | Tom Regner | Offensive Guard | Notre Dame |
Question:
What is the position of john charles?
Answer:
Defensive Back