fun main(args: Array<String>){
  val n: Long = readLine()!!.toLong()
  print(if(n%2==1L) 0 else f(n))
}

fun f(x: Long) : Long{
  var ret: Long = 0
  var k: Long = 10
  while(k <= x){
    ret += x/k
    k *= 5
  }
  return ret
}
