CREATE TABLE table_name_99 (gold INTEGER, bronze VARCHAR(255), silver VARCHAR(255), total VARCHAR(255))
What is the average number of gold medals won among participants that won less than 13 silver, less than 8 bronze, and more than 18 medals overall?
    
SQL: SELECT AVG(gold) FROM table_name_99 WHERE silver < 13 AND total > 18 AND bronze < 8