How many of the male patients are allergic to house dust mites?
male patients refer to PATIENT where gender = 'M'; allergic to house dust mites refers to allergies where DESCRIPTION = 'House dust mite allergy';
SELECT COUNT(DISTINCT T1.patient) FROM patients AS T1 INNER JOIN allergies AS T2 ON T1.patient = T2.PATIENT WHERE T2.DESCRIPTION = 'House dust mite allergy' AND T1.gender = 'M'