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:
| Position | Club | Played | Wins | Draws | Losses | Goals for | Goals against | Points | Goal Difference |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 1.0 | Litex Lovech | 30.0 | 21.0 | 6.0 | 3.0 | 73.0 | 25.0 | 69.0 | 38.0 |
| 2.0 | Levski Sofia | 30.0 | 19.0 | 7.0 | 4.0 | 73.0 | 27.0 | 64.0 | 36.0 |
| 3.0 | CSKA Sofia | 30.0 | 18.0 | 7.0 | 5.0 | 71.0 | 29.0 | 61.0 | 38.0 |
| 4.0 | Neftochimic Burgas | 30.0 | 17.0 | 4.0 | 9.0 | 59.0 | 31.0 | 55.0 | 28.0 |
| 5.0 | Slavia Sofia | 30.0 | 15.0 | 9.0 | 6.0 | 51.0 | 30.0 | 54.0 | 21.0 |
| 6.0 | Levski Kyustendil | 30.0 | 15.0 | 1.0 | 14.0 | 45.0 | 40.0 | 46.0 | 5.0 |
| 7.0 | Spartak Varna | 30.0 | 12.0 | 6.0 | 12.0 | 42.0 | 36.0 | 42.0 | 8.0 |
| 8.0 | Minyor Pernik | 30.0 | 12.0 | 5.0 | 13.0 | 32.0 | 35.0 | 41.0 | -3.0 |
| 9.0 | Lokomotiv Sofia | 30.0 | 11.0 | 6.0 | 13.0 | 42.0 | 40.0 | 39.0 | 2.0 |
| 10.0 | Metalurg Pernik | 30.0 | 11.0 | 4.0 | 15.0 | 28.0 | 36.0 | 37.0 | -8.0 |
| 11.0 | Botev Plovdiv | 30.0 | 11.0 | 3.0 | 16.0 | 35.0 | 48.0 | 36.0 | -13.0 |
| 12.0 | Dobrudzha Dobrich | 30.0 | 11.0 | 3.0 | 16.0 | 33.0 | 55.0 | 36.0 | -12.0 |
| 13.0 | Lokomotiv Plovdiv | 30.0 | 11.0 | 3.0 | 16.0 | 31.0 | 58.0 | 36.0 | -27.0 |
| 14.0 | Olimpik Teteven | 30.0 | 11.0 | 2.0 | 17.0 | 26.0 | 47.0 | 35.0 | -21.0 |
| 15.0 | Spartak Pleven | 30.0 | 7.0 | 0.0 | 23.0 | 32.0 | 75.0 | 21.0 | -43.0 |
| 16.0 | Etar Veliko Tarnovo | 30.0 | 4.0 | 2.0 | 24.0 | 21.0 | 82.0 | 14.0 | -61.0 |
Question:
What is the average goal difference using slavia sofia club?
Answer:
21.0