HDU-1465 不容易系列之一

#include <bits/stdc++.h>
#define pb push_back
#define _for(i,a,b) for(int i = (a);i < (b);i ++)
#define INF 0x3f3f3f3f

using namespace std;

const int maxn = 5003;

long long solve(long long n)
{
    if(n==1)
        return 0;
    else if(n==2)
        return 1;
    
    return (n-1)*(solve(n-1)+solve(n-2));
}

int main()
{
    int N;
    while(~scanf("%d",&N))
    {
        cout << solve(N) << endl;
    }
    return 0; 
}