import java.util.Scanner

object Main extends App {
  val sc = new Scanner(System.in)

  val n = sc.nextLong

  val res =
    if( n % 2 != 0) 0
    else {
      (1 to 26).foldLeft(0L){ case (sum, i) =>
        sum + (n / Math.pow(5, i).toLong) / 2L
      }
    }

  println(res)

}
