None of them match. On the other hand, there is at least one pair, so one pair is regarded as a pair. The legend is: 1 1+23+45+67+89, so ***A99 is 9 * 8 * 7 * 6 * 5 * 4 * 3 * 2 * 65438.

That is, the number of successful pairing at least once is ***A99+4.

So the unpaired number is: a10/kloc-0-a99, that is, (10 * 9 * 8 * 7 * 6 * 5 * 4 * 3 * 2 *1)-(9 * 8 * 7 * 6.