program main
  implicit none
  character(100000) :: s
  logical :: appear(26)
  integer :: i, j
  read(*, *) s
  appear(:) = .false.
  do i = 1, len(trim(s))
    j = ichar(s(i:i)) - ichar("a") + 1
    appear(j) = .true.
  end do
  do i = 1, 26
    if (.not. appear(i)) then
      write(*, "(a)") char(i + ichar("a") - 1)
      stop 0
    end if
  end do
  write(*, "(a)") "None"
end program main
