program ABC_063_B_Varied
    implicit none
	character(26) S
	integer i, j
	read(*, *) S
	do j = 1, len_trim(S)
	    do i = j + 1, len_trim(S)
		    if(S(j:j) == S(i:i)) then
			    write(*, *) 'no'
				goto 1
			end if
		end do
	end do
	write(*, *) 'yes'
1 continue
end program ABC_063_B_Varied