The altitude of A is 60m, the altitude of B is 20m, the altitude of C is -30m, and the altitude of D is -10m. Which place is the highest? Where is the lowest? How many meters is the highest place higher than the lowest place?
甲地海拔是60m，乙地海拔是20m，丙地海拔是-30m，丁地海拔是-10m，哪个地方最高？哪个地方最低？最高的地方比最低的地方高多少米？
<gadget id="calculator">60 - (-30)</gadget>
<output>90</output>

<result>90</result>