Xiao Ming's father dug a rectangular vegetable field by the river, 9 meters long and 4 meters wide. He used fences to enclose the three sides that are not near the river. How many meters of fences should be used at least?
小明的爸爸在河边挖了一块长方形菜地，长9米，宽4米．他用篱笆把不靠河边的三面围了起来，至少得用多少米篱笆？
<gadget id="calculator">4 * 2</gadget>
<output>8</output>

<gadget id="calculator">9 + 8</gadget>
<output>17</output>

<result>17</result>