4 good friends take pictures at tourist attractions (regardless of the order of stops), how many different ways of taking pictures?
4个好朋友在旅游景点拍照留念(不考虑站的顺序)，共自多少种不同的照法？
<gadget id="calculator">4 * 3 * 2 * 1</gadget>
<output>24</output>

<result>24</result>