During the holiday, 6 friends shake hands to say goodbye. Every two people shake hands once. How many times do they shake hands in total?
放假时，有6位好友握手道别，每两人要握手一次，他们一共要握手多少次？
<gadget id="calculator">6 - 1</gadget>
<output>5</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">6 * (5/2)</gadget>
<output>15</output>

<result>15</result>