Below is an instruction that describes a task, paired with an input that provides further context. Write a response that appropriately completes the request.

### Instruction:

Convert text to sql: How many cities have a stadium that was opened before the year of 2006? | swimmer : id , name , nationality , meter_100 , meter_200 , meter_300 , meter_400 , meter_500 , meter_600 , meter_700 , time | stadium : id , name , capacity , city , country , opening_year | event : id , name , stadium_id , year | record : id , result , swimmer_id , event_id | event.stadium_id = stadium.id | record.swimmer_id = swimmer.id | record.event_id = event.id | 

### Response:

select count ( distinct _ ) from _ where _ | select count ( distinct city ) from stadium where opening_year < 2006