7・112 A set of five-volume encyclopedias is arranged in ascending order on a bookshelf, i.e., from left to right, from Volume 1 to Volume 5. Now, we want to rearrange them in descending order, i.e., from left to right, from Volume 5 to Volume 1, but each time only the positions of two adjacent volumes can be swapped. What is the minimum number of such swaps needed to achieve the goal?