program main
  implicit none
  character(26) :: s
  integer :: i, j, n
  read(*, *) s
  n = len(trim(s))
  do i = 1, n
    do j = i + 1, n
      if (s(i:i) == s(j:j)) then
        write(*, "(a)") "no"
        stop 0
      end if
    end do
  end do
  write(*, "(a)") "yes"
end program main
