18 football teams play a single-elimination match, how many matches must be played to produce a champion.
18支足球队进行单场淘汰制比赛，要进行多少场比赛才能产生冠军．
<gadget id="calculator">18 / 2</gadget>
<output>9</output>

<gadget id="calculator">8 / 2</gadget>
<output>4</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">9 + 4 + 2 + 1 + 1</gadget>
<output>17</output>

<result>17</result>