P1595 Envelope Problem

Someone writes $n$ letters and has $n$ envelopes. If all letters are put into the wrong envelopes, how many different ways are there in total?

A single integer $n$, the number of envelopes, with $n \le 20$.

A single integer representing the number of ways.

For $100\%$ of the testdata, $1 \le n \le 20$. Translated by ChatGPT 5