user: Question: What is the sum of silver medals of the nation cuba, which has less than 0 gold?
Columns: ['Rank', 'Nation', 'Gold', 'Silver', 'Bronze', 'Total']
Types: ['text', 'text', 'real', 'real', 'real', 'real']
Sample row: ['1', 'Soviet Union', 12.0, 4.0, 4.0, 20.0]
SQL:

assistant: SELECT SUM("Silver") FROM "Table" WHERE "Nation" = 'cuba' AND "Gold" < 0.0;