require 'pry'
S=gets.chomp

h = {}
97.upto(97+12) do |i|
  h[i.chr] = 0
end
(S.length).times do |i|
  if !h.key?(S[i])
    puts 'Impossible'
    exit
  end
  h[S[i]] = h[S[i]] + 1
end
found = false
h.each do |k,v|
  if v > 2 then
    puts 'Impossible'
    found = true
    break
  elsif v == 0
    puts k
    found = true
    break
  end
end

if !found
  h.each do |k,v|
    puts k
  end
end