#include<iostream>
#include<math.h>
#include<vector>
#include<algorithm>
#include<cstdio>
using namespace std;

int main(){
    int n;
    const int max = 50000;
    int a[max];
    a[1] = 0;
    for(int i = 2; i < max; i++) a[i] = 1;
    for(int i = 2; i * i <= max; i++){
        for(int j = 2; j*i <= max; j++){
            a[i*j] = 0;
        }
    }
    while(cin >> n){
        if(n == 0) break;
        int count = 0;
        if(n % 2 == 1){
            if(a[n-2]){
                cout << 1 << endl;
            }else{
                cout << 0 << endl;
            }
        }else{
            for(int i = 2; i <= n-i; i++)
                if(a[i] && a[n-i])
                    count++;
            cout << count <<endl;
        }
    }
    return 0;
}
