program varied
  implicit none
  character(26) :: s
  integer       :: n, i, m(26), c
  read(*,*) s
  s = adjustl(s)
  n = len_trim(s)
  m = 0
  do i = 1, n
    c = ichar(s(i:i)) - 96
    m(c) = m(c) + 1
  end do
  if maxval(m) > 1 then
    write(*,'(a)') "no"
  else
    write(*,'(a)') "yes"
  end if
end program varied