1 #include 
     
       2 #define pb push_back 3 #define _for(i,a,b) for(int i = (a);i < (b);i ++) 4 #define INF 0x3f3f3f3f 5  6 using namespace std; 7  8 const int maxn = 5003; 9 10 long long solve(long long n)11 {12     if(n==1)13         return 0;14     else if(n==2)15         return 1;16     17     return (n-1)*(solve(n-1)+solve(n-2));18 }19 20 int main()21 {22     int N;23     while(~scanf("%d",&N))24     {25         cout << solve(N) << endl;26     }27     return 0; 28 }