program prob2
    implicit none
    character(len=30)::s
    integer::c(26), i, l, tmp
    c = 0
    read(*,*) s
    l = len_trim(s)
    do i = 1, l
        tmp = ichar(s(i:i)) - ichar('a') + 1
        if(c(tmp) == 1) then
            write(*,'(a)') "no"
            stop
        else
        	c(tmp) = 1
        end if
    end do
    write(*,'(a)') "yes"
    stop
end program