user: Question: What is the average goal difference using Slavia Sofia club?
Columns: ['Position', 'Club', 'Played', 'Wins', 'Draws', 'Losses', 'Goals for', 'Goals against', 'Points', 'Goal Difference']
Types: ['real', 'text', 'real', 'real', 'real', 'real', 'real', 'real', 'real', 'real']
Sample row: [1.0, 'Litex Lovech', 30.0, 21.0, 6.0, 3.0, 73.0, 25.0, 69.0, 38.0]
SQL:

assistant: SELECT AVG("Goal Difference") FROM "Table" WHERE "Club" = 'Slavia Sofia';