错排问题

#include<bits/stdc++.h>
typedef long long LL;
using namespace std;
int n;
LL a[100]; 
int main ()
{
	cin >> n;
	//错排
	a[0] = 1;
	a[1] = 0;
	for(int i = 2;i <= n;++ i)
	{
		a[i] = (i - 1) * (a[i - 1] + a[i - 2]);
	} 
	cout << a[n];
	return 0;
}