import scala.io.StdIn.readInt

object Main {
  def sieve(n:Int) = {
    val primes = Array.tabulate(n+1) { _ % 2 }
    primes(1) = 0
    primes(2) = 1

    for( i <- 3 to Math.sqrt(n).toInt by 2 )
      if(primes(i) == 1)
        (i*i to n by i*2).foreach { primes(_) = 0 }
    primes
  }

  def main(args:Array[String]) = {
    val p = sieve(50000)
    var n = 0
    while( { n = readInt; n} != 0 ) {
      var cnt = 0
      if(p(2) == 1 && p(n-2) == 1) cnt += 1
      for(i<- 3 to n/2 by 2)
        if(p(i) ==1 && p(n-i) ==1)
          cnt += 1
      println(cnt)
    }
  }
}