#include<iostream>
#include<string>
#include<algorithm>
#include<map>
#include<set>
#include<utility>
#include<vector>
#include<cmath>
#include<cstdio>
#define loop(i,a,b) for(int i=a;i<b;i++) 
#define rep(i,a) loop(i,0,a)
#define pb push_back
#define mp make_pair
#define it ::iterator
#define all(in) in.begin(),in.end()
const double PI=acos(-1);
const double ESP=1e-10;
using namespace std;
int main(){
#define MAX 1000001
  bool num[MAX]={1,1,0};
  for(int i=3;i<MAX;i+=2){
    if(num[i])continue;
    for(int j=i*3;j<MAX;j+=i*2){
      num[j]=1;
    }
  }
  int n;
  while(cin>>n,n){
    int co=0;
    for(int i=3;i<=n/2;i+=2){
      if(!num[i]&&!num[n-i])co++;
    }
    cout<<co<<endl;
  }


}