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

### Instruction:
How long does the one that stars with son bou of middle difficulty, and on a stage number larger than 13 last?

### Inputs:
table_name: table, columns: ['Stage', 'Start', 'Arrival', 'Duration', 'Difficulty']

### Response:
SELECT Duration FROM table WHERE Stage > 13 AND Difficulty = middle AND Start = son bou