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

### Instruction:
How many total units were built of Model ds-4-4-660 with a b-b wheel arrangement and a PRR Class of bs6?

### Inputs:
table_name: table, columns: ['PRR Class', 'Builder’s Model', 'Build date', 'Total produced', 'Wheel arrangement', 'Service', 'Power output']

### Response:
SELECT SUM Total produced FROM table WHERE Wheel arrangement = b-b AND PRR Class = bs6 AND Builder’s Model = ds-4-4-660