Xiaohua's and Xiaoqing's homes are in the west of the school. Xiaohua's home is 400 meters away from the school, and Xiaoqing's home is 900 meters away from the school. How many meters are there between their two families?
小华和小青家同在学校的西面，小华家离学校400米，小青家离学校900米，她们两家相距多少米．
<gadget id="calculator">900 - 400</gadget>
<output>500</output>

<result>500</result>