Write SQL query to solve given problem: Among the patients with hypertension, what is the average of their diastolic blood pressure?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT AVG(T1.VALUE) FROM observations AS T1 INNER JOIN patients AS T2 ON T1.PATIENT = T2.patient INNER JOIN conditions AS T3 ON T2.patient = T3.PATIENT WHERE T3.DESCRIPTION = 'Hypertension' AND T1.DESCRIPTION = 'Diastolic Blood Pressure'