题解

#include<bits/stdc++.h>
using namespace std;
int main(){
    long long n,a[25];
    cin>>n;
	a[1]=0;
	a[2]=1;
	for(long long i=3;i<=n;++i)
        a[i]=(i-1)*(a[i-1]+a[i-2]);
	cout<<a[n];
}