#include <iostream>
#define X 50000
using namespace std;

int gold(int n,bool p[X+1]){
	if(n%2==1){
		if(p[n-2]) return 1;
		else return 0;
	}
	else{
		int c = 0;
		for(int i=2;i<=X;i++){
			if(n-i<i) break;
			if(p[i] and p[n-i]) c++;
		}
		return c;
	}
}

int main(){
	int n;
	bool p[X+1];
	for(int i=0;i<=X;i++) p[i] = true;
	for(int i=2;i<=X;i++){
		if(not p[i]) continue;
		for(int j=2;j<=X/i;j++) p[i*j] = false;
	}
	while(1){
		cin >> n;
		if(n==0) break;
		cout << gold(n,p) << endl;
	}
}