implicit none
character(100000) :: s
integer :: i,n
logical :: a(97:122)

read *,s
n = len_trim(s)

a(:) = .false.
do i = 1, n
   a(ichar(s(i:i))) = .true.
enddo

do i = 97, 122
   if (.not. a(i)) then
      print '(a)',char(i)
      stop
   endif
enddo

print '(a)','None'
stop
end