4 good friends go to take pictures, if every two good friends have to take a picture, how many pictures will be taken in total.
4个好朋友去照相，如果每两个好朋友都要拍一张照片，一共需要拍多少张照片．
<gadget id="calculator">3 + 2 + 1</gadget>
<output>6</output>

<result>6</result>