There are 5 classmates, and each of them will give each other a gift. How many gifts should they give in total?
有5个同学，他们每两人互相送一件礼物，一共要送多少件礼物？
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<result>20</result>