There are currently 20 teachers in the mathematics department of Guangdong Beijiang Experimental School. One of them is the principal. He does not lead a class. The other two teachers only teach one class. If the number of classes in the three grades is the same, how many classes are there in the ninth grade?
广东北江实验学校数学科组现有教师20人，其中有一位是校长，他没有带班上课，另有两位老师每人只上一个班的课，其余老师均上两个班的课，我校三个年级的班数一样，则九年级的班数有多少个．
<gadget id="calculator">2 * 1</gadget>
<output>2</output>

<gadget id="calculator">20 - 1 - 2</gadget>
<output>17</output>

<gadget id="calculator">17 * 2</gadget>
<output>34</output>

<gadget id="calculator">2 + 34</gadget>
<output>36</output>

<gadget id="calculator">36 / 3</gadget>
<output>12</output>

<result>12</result>