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

<result>50</result>