1 /* 2 http://codeforces.com/problemset/problem/449/C 3 cf 449 C. Jzzhu and Apples 4 数论+素数+贪心 5 */ 6 #include <cstdio> 7 #include <algorithm> 8 using namespace std; 9 const int Nmax=100005; 10 int is_prime[Nmax]; 11 int book[Nmax]; 12 int cnt[Nmax]; 13 int n,ans; 14 void get__prime() 15 { 16 for(int i=2;i<=n;i++) 17 is_prime[i]=1; 18 for(int i=2;i<=n;i++) 19 { 20 if(is_prime[i]) 21 { 22 for(int j=2;j*i<=n;j++) 23 { 24 is_prime[i*j]=0; 25 } 26 } 27 } 28 } 29 30 int main() 31 { 32 //freopen("cf449C.in","r",stdin); 33 scanf("%d",&n); 34 get__prime(); 35 for(int i=n;i>=2;i--) 36 { 37 if(is_prime[i]) 38 { 39 cnt[i]=1; 40 for(int j=2;i*j<=n;j++) 41 { 42 if(!book[i*j]) 43 { 44 cnt[i]++; 45 book[i*j]=1; 46 } 47 } 48 if(cnt[i]&1) 49 book[2*i]=0; 50 ans+=cnt[i]>>1; 51 } 52 } 53 printf("%d ",ans); 54 for(int i=1;i<=n;i++) 55 book[i]=0; 56 for(int i=n;i>=2;i--) 57 { 58 if(is_prime[i] && cnt[i]>1) 59 { 60 if(cnt[i]&1) 61 { 62 int flag=1; 63 for(int j=1;i*j<=n;j++) 64 { 65 if(j==2) 66 continue; 67 if(!book[i*j]) 68 { 69 printf("%d",i*j); 70 if(flag) 71 printf(" "); 72 else 73 printf(" "); 74 flag^=1; 75 book[i*j]=1; 76 } 77 } 78 } 79 else 80 { 81 int flag=1; 82 for(int j=1;i*j<=n;j++) 83 { 84 if(!book[i*j]) 85 { 86 printf("%d",i*j); 87 if(flag) 88 printf(" "); 89 else 90 printf(" "); 91 flag^=1; 92 book[i*j]=1; 93 } 94 } 95 } 96 } 97 } 98 return 0; 99 }