P8637 [Lanqiao Cup 2016 NOI Qualifier B] Swapping Bottles

There are $N$ bottles, numbered $1 \sim N$, placed on a shelf. For example, there are $5$ bottles: $$2,1,3,5,4$$ Each time, you are allowed to pick up $2$ bottles and swap their positions. After several swaps, make the bottle numbers become: $$1,2,3,4,5$$ For such a simple case, it is obvious that at least $2$ swaps are needed to restore the order. What if there are more bottles? You can solve it by programming.

The first line: a positive integer $N$ ($N

Output one positive integer in a single line, representing the minimum number of swaps needed to finish the sorting.

Time limit: 1 second, 256M. Lanqiao Cup 2016, the 7th provincial contest. Lanqiao Cup 2016 provincial contest, Group B, Problem I. Translated by ChatGPT 5