#include<iostream>
using namespace std;

const int size = 40000;
int a[size];
int n;

void getPrime();
 
int solve();

int main()
{
	getPrime();
	 
	while ( true )
	{
		cin >> n;

		if ( n ==  0 )
			break;

		cout << solve() << endl;
	}
	return 0;
}

void getPrime()
{
	for ( int i = 2; i < size; i ++)
		a[i] = 1;

	for ( int j = 2; j < size; j ++ )
	{
		if (a[j])
			for ( int k = j + j; k < size; k += j )
				a[ k ] = 0;
	}
}

int solve()
{
	int c = 0;
	for ( int i = 2; i <= n / 2; i ++ )
		if ( a[i] && a[ n - i ])
			c ++;

	return c;
}