Grandpa has a rectangular vegetable field with a length of 15 meters and a width of 10 meters. At least how long a fence needs to be prepared to enclose the vegetable field.
爷爷有一块长15米，宽10米的长方形菜地，最少需要准备多长的篱笆才能把菜地都围起来．
<gadget id="calculator">15 + 10</gadget>
<output>25</output>

<gadget id="calculator">25 * 2</gadget>
<output>50</output>

<result>50</result>