1 #include<cstdio>
 2 
 3 using namespace std;
 4 
 5 int n,ans;
 6 bool vis[8000001]; 
 7 
 8 int main()
 9 {
10     scanf("%d",&n);
11     for(int i = 2;i * i <= n; i++)
12         if(!vis[i]) {
13             for(int j = i * i;j <= n; j += i)
14                 vis[j] = 1;    
15         }
16     for(int i = 2;i <= n / 2; i++)
17         if(!vis[i] && !vis[n-i]) ans++;
18     printf("%d",ans);
19     return 0;
20 }