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: Return the average enrollment of universities founded before 1850. | basketball_match : team_id , school_id , team_name , acc_regular_season , acc_percent , acc_home , acc_road , all_games , all_games_percent , all_home , all_road , all_neutral | university : school_id , school , location , founded , affiliation , enrollment , nickname , primary_conference | basketball_match.school_id = university.school_id | 

### Response:

select avg ( _ ) from _ where _ | select avg ( enrollment ) from university where founded < 1850