#include <cstdio>

#define MAX 1000000

bool prime[MAX];

void make(){
  fill(prime,prime+MAX,true);
  prime[0] = prime[1] = false;

  for(int i = 2 ; i*i < MAX ; i++){
    if(prime[i]){
      for(int j = 2*i ; j < MAX ; j += i){
	prime[j] = false;
      }
    }
  }
}

int main(){
  make();
  int n;
  while(scanf("%d" ,&n),n){
    int cnt = 0;
    for(int i = 2 ; i+i <= n ; i++){
      if(prime[i] && prime[n-i]){
	cnt++;
      }
    } 
    printf("%d\n" ,cnt);
  }

  return 0;
}