instruction: provide the number of patients on nifedipine medication who have been diagnosed with primary malignant neoplasm of liver.
response: SELECT COUNT(DISTINCT demographic.subject_id) FROM demographic INNER JOIN diagnoses ON demographic.hadm_id = diagnoses.hadm_id INNER JOIN prescriptions ON demographic.hadm_id = prescriptions.hadm_id WHERE diagnoses.long_title = "Malignant neoplasm of liver, primary" AND prescriptions.drug = "NIFEdipine"