The 35 students in the class are lined up. Counting from the left, Xiaohong is the 20th, and counting from the right, Xiaogang is the 21st. How many students are there between Xiaohong and Xiaogang?
全班35名学生排成一行，从左边数，小红是第20位，从右边数，小刚是第21位，小红与小刚中间间隔着多少名同学．
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<gadget id="calculator">35 - 19</gadget>
<output>16</output>

<gadget id="calculator">21 - 16 - 1</gadget>
<output>4</output>

<result>4</result>