1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstring>
 4 #include<cmath>
 5 #include<algorithm>
 6 #define mem(x,y) memset(x,y,sizef(x))
 7 using namespace std;
 8 typedef long long LL;
 9 const int INF=0x3f3f3f3f;
10 
11 int main(){
12     int n;
13     LL ans[21];
14     ans[1]=0;ans[2]=1;
15     for(int i=3;i<=20;i++)
16     ans[i]=(i-1)*(ans[i-1]+ans[i-2]);
17     while(~scanf("%d",&n))printf("%I64d\n",ans[n]);
18     return 0;
19 }

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstring>
 4 #include<cmath>
 5 #include<algorithm>
 6 #define mem(x,y) memset(x,y,sizef(x))
 7 using namespace std;
 8 typedef long long LL;
 9 const int INF=0x3f3f3f3f;
10 LL fac[21];
11 int main(){
12     int n;
13     fac[0]=1;
14     for(int i=1;i<=20;i++)fac[i]=fac[i-1]*i;
15     while(~scanf("%d",&n)){
16         LL ans=0;
17     for(int i=0;i<=n;i++){
18         if(i&1)ans-=fac[n]/fac[i];
19         else ans+=fac[n]/fac[i];
20         }
21         printf("%I64d\n",ans);
22     }
23     return 0;
24 }