#!/usr/bin/env ruby
N = 50_000 # ??¢?´¢???????´???°?????????
elt = Array.new(N, 1)
primes = []


# ??¨????????????????????????
elt[0] = 0
elt[1] = 0

N.times do |i|
  next if elt[i].zero?
  primes << i
  k = i * 2
  while k < N
    elt[k] = 0
    k += i
  end
end

while line = gets
  n = line.to_i
  break if n.zero?

  cnt = 0
  primes.each do |pr|
    break if pr > n/2
    cnt += 1 if primes.include?(n-pr)
  end
  puts cnt
end