Translate the following question to SQL: Return the average enrollment of universities founded before 1850.

Schema: CREATE TABLE basketball_match (
    Team_ID INT   PRIMARY KEY,
    School_ID INT,
    Team_Name TEXT,
    ACC_Regular_Season TEXT,
    ACC_Percent TEXT,
    ACC_Home TEXT,
    ACC_Road TEXT,
    All_Games TEXT,
    All_Games_Percent INT,
    All_Home TEXT,
    All_Road TEXT,
    All_Neutral TEXT,
    UNIQUE (Team_ID),
    FOREIGN KEY (School_ID) REFERENCES university(School_ID)
);

CREATE TABLE university (
    School_ID INT   PRIMARY KEY,
    School TEXT,
    Location TEXT,
    Founded REAL,
    Affiliation TEXT,
    Enrollment REAL,
    Nickname TEXT,
    Primary_conference TEXT,
    UNIQUE (School_ID)
);
user
SELECT avg(enrollment) FROM university WHERE founded  <  1850
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