6 cards, placed in four different boxes. How many different ways are there to ensure that each box is not empty?
6张卡片，放在四个不同的盒里．若要保证每个盒里都不空，共有多少种不同的放法．
<gadget id="calculator">4 + 6</gadget>
<output>10</output>

<result>10</result>