A square vegetable field has a side length of 40 decimeters. A fence should be built around it. How long is the fence?
一块正方形菜地，边长是40分米，要给它四周围上篱笆，篱笆长多少米？
<gadget id="calculator">40 * 4</gadget>
<output>160</output>

<result>160</result>