program ABC071B
    implicit none
    integer(8)::i
    integer(8)::flg=0
    integer(8),dimension(26)::L
    character(100000)::S
    L=0
    read(5,*)S

    do i=1,len_trim(S)
        L(ichar(S(i:i))-96)=1
    end do

    do i=1,26
        if(L(i)==0)then
            flg=1
            print'(A)',char(i+96)
            exit
        end if
    end do

    if(flg==0)then
        print'(A)',"None"
    end if
end program ABC071B