program abc071b
  implicit none
  integer i
  character(26),parameter :: alphabet = 'abcdefghijklmnopqrstuvwxyz'
  character(100000) s

  read *, s
  i = verify(alphabet, s)
  if (i == 0) then
     print '(a)', 'None'
  else
     print '(a)', alphabet(i:i)
  end if
end program abc071b
