program main
  implicit none
  integer(8) i, N, ans10, N5, ans5
  read(*, *) N
  N5 = N
  !read(*, *) (L(i), i = 1,N)
  ans10 = 0
  ans5 = 0
  !N5 = (N/10)/5
  if (mod(N,2) == 1) then
    ans10 = 0
  else
    do
      if (N5 == 0) then
        exit
      else
        N5 = N5/5
        ans5 = ans5 + N5/2
        !write(*, *) ans
      end if
    end do
  end if
  
  write(*, *) ans10+ans5
end program main
