program ABC063B
    implicit none
    integer(8)::i
    integer(8)::flg=0
    integer(8),dimension(26)::L
    character(26)::S
    read(5,*)S
    L=0
    do i=1,len_trim(S)
        L(ichar(S(i:i))-96)=L(ichar(S(i:i))-96)+1
        if(L(ichar(S(i:i))-96)>1)then
            flg=1
            exit
        end if
    end do

    if(flg==1)then
        print'(A)',"no"
    else
        print'(A)',"yes"
    end if
end program ABC063B