There are 4 classes in the third grade, every two classes have to tug-of-war once, so how many times in total.
三年级有4个班，每两个班要拔河一次，这样一共要拔多少次．
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

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

<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<result>6</result>