#include <iostream>

using namespace std;

const int Max = 1000000;
bool P[Max];

void eratos()
{
	for( int i = 0;i < Max;i++ ) P[i] = true;

	P[0] = P[1] = false;

	for( int i = 0;i < Max;i++ ) if( P[i] == true ) for( int j = 2;i * j < Max;j++ ) P[i * j] = false;
}


int main(int argc, char const* argv[])
{
	int n;

	eratos();

	while( cin >> n && n != 0 ){
		int count = 0;

		for( int i = 2;i <= n/2;i++ ) if( P[i] ) if( P[n - i] ) count++;

		cout << count << endl;
	}
	return 0;
}