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

### Instruction:
What is Population density (per km²), when Name is Antigua and Barbuda, and when Capital is St. John's?

### Inputs:
table_name: table, columns: ['Name', 'Subdivisions', 'Area (km²)', 'Population (1 July 2005 est.)', 'Population density (per km²)', 'Capital']

### Response:
SELECT Population density (per km²) FROM table WHERE Name = antigua and barbuda AND Capital = st. john's