The altitude of A is -30 meters, and the altitude of B is 20 meters. What is the difference in altitude between A and B?
甲地的海拔高度为-30米，乙地的海拔高度为20米，甲乙两地海拔高度相差多少米．
<gadget id="calculator">20 - (-30)</gadget>
<output>50</output>

<result>50</result>