program linked_list
  implicit none
  type node
    type (node), pointer :: next_node
    integer value
  end type node
  integer num
  type (node), pointer :: first_node, new_node, p
  
  nullify (first_node)

  do
    print *, 'Input a positive number: (Enter negative number to end)'
    read *, num
    if (num<0) exit

    ! リストの最初に新しい値を追加
    allocate (new_node)
    new_node%next_node => first_node
    new_node%value = num
    first_node => new_node

  end do

  p => first_node
  print *, "================================"
  do while (associated(p))
    print *, p%value
    p => p%next_node
  end do

end program linked_list