经典的信封装信问题
f[ n ] = ( n-1 ) * ( f[ n-1 ]+f[ n-2 ] )
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
typedef long long int64;
const int maxn = ;
int64 a[ maxn ]; void init(){
a[ ] = a[ ] = ;
a[ ] = ;
for( int i=;i<=;i++ ){
a[ i ] = ( i- )*(a[ i- ] + a[ i- ]);
}
return ;
} int main(){
init();
int n;
while( scanf("%d",&n)== ){
printf("%I64d\n",a[ n ]);
}
return ;
}