A patient infected with H7N9 was hospitalized on April 3 and was cured and discharged on April 21. How many days did this patient spend in hospital?
一位病人感染H7N9于4月3日入院治疗，4月21日治愈出院，这位病人一共住院多少天？
<gadget id="calculator">21 - 3</gadget>
<output>18</output>

<result>18</result>