There are 6 table tennis players, if every two players shake hands once, how many handshakes are needed in total?
有6名乒乓球运动员，如果每两名运动员之间都要握一次手，一共要握多少次手．
<gadget id="calculator">6 - 1</gadget>
<output>5</output>

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

<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<result>15</result>