Table schema:
CREATE TABLE table_name_51 (gold INTEGER, bronze VARCHAR, nation VARCHAR, silver VARCHAR, total VARCHAR)

Question: Silver larger than 0, and a Total smaller than 3, and a Nation of bulgaria, and a Bronze smaller than 0 had what sum of gold?
SQL: SELECT SUM(gold) FROM table_name_51 WHERE silver > 0 AND total < 3 AND nation = "bulgaria" AND bronze < 0