program not_found
  implicit none
  character(100000) :: s
  integer           :: n, i, a(26), c
  a = 0
  read(*,*) s
  s = adjustl(s)
  n = len_trim(s)
  do i = 1, n
    c = ichar(s(i:i)) - 96
    a(c) = a(c) + 1
  end do
  do i = 1, 26
    if (a(i).eq.0) then
      write(*,'(a)') achar(i+96)
      stop
    end if
  end do
  write(*,'(a)') "None"
  stop
end program not_found