CREATE TABLE table_33105 ("Position" FLOAT, "Club" TEXT, "Played" FLOAT, "Wins" FLOAT, "Draws" FLOAT, "Losses" FLOAT, "Goals for" FLOAT, "Goals against" FLOAT, "Points" FLOAT, "Goal Difference" FLOAT)

What is the average goal difference using slavia sofia club?
SELECT AVG("Goal Difference") FROM table_33105 WHERE "Club" = 'slavia sofia'
SELECT AVG("Goal Difference") FROM "table_33105" WHERE "Club" = 'slavia sofia'