set of five-volume encyclopedias is arranged on the shelves in ascending order, that is, from left to right, from volume 1 to volume 5. Now I want to change them to be placed in descending order, that is, from left to right from the fifth volume to the first volume, but only the positions of the two adjacent volumes are allowed to be exchanged each time. Ask how many times this kind of exchange should be done at least to achieve the goal? Why?
一套五卷百科全书按递增顺序摆放在书架上，即从左到右由第1卷依次排到第5卷．现想把它们改换成按递减顺序摆放，即从左到右由第5卷依次排到第1卷，但每次只允许交换相邻摆放的两卷的位置．问最少要做多少次这种交换才能达到目的？为什么？
<gadget id="calculator">4 + 3 + 2 + 1</gadget>
<output>10</output>

<result>10</result>