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:
|  | Name | Title Reign | Title Recognition | Successful Defenses |
| --- | --- | --- | --- | --- |
| 1. | Joe Calzaghe | 10 years, 11 months, 15 days | IBF, WBA, WBO, WBC, The Ring | 21 |
| 2. | Sven Ottke | 5 years, 5 months, 3 days | IBF, WBA (Super) | 21 |
| 3. | Frankie Liles | 4 years, 10 months, 0 days | WBA | 8 |
| 4. | Lucian Bute | 4 years, 7 months, 7 days | IBF | 9 |
| 5. | Chris Eubank | 4 years, 4 months, 0 days | WBO | 14 |
| 6. | Andre Ward | 4 years, 6 months, 3 weeks and 1 day | WBA (Super), WBC, The Ring | 6 |
| 7. | Chong-Pal Park | 3 years, 7 months, 17 days | IBF, WBA | 10 |
| 8. | Nigel Benn | 3 years, 4 months, 28 days | WBC | 9 |
| 9. | Robert Stieglitz | 3 years, 0 months, 3 days | WBO | 6 |
| 10. | Mikkel Kessler | 2 years, 11 months, 23 days | WBA (Super) | 4 |
Question:
what is the difference between the number of successful defenses by joe calzaghe and robert stieglitz?
Answer:
15