What is High Rebounds, when Location Attendance is "Madison Square Garden Unknown", and when Date is "May 18"?
CREATE TABLE "playoffs" (
  "game" real,
  "date" text,
  "team" text,
  "score" text,
  "high_points" text,
  "high_rebounds" text,
  "high_assists" text,
  "location_attendance" text,
  "series" text
);
SELECT "high_rebounds" FROM "playoffs" WHERE "location_attendance"='madison square garden unknown' AND "date"='may 18';