In the third grade, 20 people subscribed to "Last Year Tiandi", 18 people subscribed to Fairy Tale World, and 8 people subscribed to both magazines. How many people subscribed to these two magazines in total?
三年级订《上年天地》的有20人，订童话世界有18人，两种杂志都订的有8人，订这两种杂志的一共有多少人？
<gadget id="calculator">18 - 8</gadget>
<output>10</output>

<gadget id="calculator">20 + 10</gadget>
<output>30</output>

<result>30</result>