This of course shows that we can at least find that the number of even permutations is equal to the number of odd permutations, that is, s ≤ t.

In the same way, exchange the first two digits of any even-numbered arrangement, and you can get an odd-numbered arrangement.

This shows that any even permutation can also find its corresponding odd permutation.

This also shows that we can at least find that the number of odd permutations is equal to the number of even permutations, that is, t ≤ x.

Then if s≤t and t≤x are both true, of course, it can only be s = t.