Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
What is the sum of Silver when the total is less than 6, the rank is 6 and the Bulgaria is the nation?

### Inputs:
table_name: table, columns: ['Rank', 'Nation', 'Gold', 'Silver', 'Bronze', 'Total']

### Response:
SELECT SUM Silver FROM table WHERE Total < 6 AND Rank = 6 AND Nation = bulgaria