implicit none
character(26) :: s
integer :: i,j

read *,s
do i = 1, len_trim(s)-1
   do j = i+1, len_trim(s)
      if (s(i:i) .eq. s(j:j)) then
         write(*,'(a)') 'no'
         stop
      endif
   enddo
enddo
write(*,'(a)') 'yes'
end