There are a total of 4 students in Ding Ding's group, every two students shook hands once, how many times did they shake hands in total?
丁丁的小组里有共4个同学，每两个同学之间握一次手，一共握了多少次手．
<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>