How many knots should be tied on a 20 meter long rope with knots every 2 meters and no knots at both ends?
在一条长20米的绳子上，每隔2米打一个结，两端不打结，一共应打多少个结．
<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<result>9</result>