4 children shake hands. If every two of them shake hands with each other, how many times do they shake hands in total?
4个小朋友握手，如果他们每两个人之间都要相互握一次手，他们4人之间一共要握手多少次．
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">3 / 2</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">4 * (3/2)</gadget>
<output>6</output>

<result>6</result>