#define _USE_MATH_DEFINES
#define INF 0x3f3f3f3f
#include <cstdio>
#include <iostream>
#include <sstream>
#include <cmath>
#include <cstdlib>
#include <algorithm>
#include <queue>
#include <stack>
#include <limits>
#include <map>
#include <string>
#include <cstring>
#include <set>
#include <deque>
#include <bitset>
#include <list>
#include <cctype>
#include <utility>

using namespace std;

typedef long long ll;
typedef pair <int,int> P;
typedef pair <int,P > PP;

int tx[] = {0,1,0,-1};
int ty[] = {-1,0,1,0};

static const double EPS = 1e-8;

bool isPrime[1000001];

int main(){

	memset(isPrime,1,sizeof(isPrime));
	isPrime[0] = false;
	isPrime[1] = false;

	for(int i=2;i*i<=1000000;i++){
		if(isPrime[i]){
			for(int j=i+i;j<=1000000;j+=i){
				isPrime[j] = false;
			}
		}
	}
	int n;
	while(~scanf("%d",&n)){
		if(n==0) break;
		int res = 0;
		for(int i=2;i<=n;i++){
			if(n-i < i) break;
			if(isPrime[i] && isPrime[n-i]) res++;
		}

		printf("%d\n",res);
	}
}