19 people lined up for the long jump. The 17th from front to back is Xiaohong, and the 9th is Xiaoshan. How many classmates are there between Xiaohong and Xiaoshan? column calculation.
19个人排队跳远，从前往后数第17位是小红，第9位是小山，请问小红和小山之间有几位同学？列式计算．
<gadget id="calculator">17 - 9 - 1</gadget>
<output>7</output>

<result>7</result>