S = gets.chomp

counter = Hash.new(0)

S.chars.sort.each do |ch|
  counter[ch] += 1
end

corrects = []
base = 'abcdefghijklm'

base.chars.each do |ch|
  cand = (base + ch).chars.sort.join
  corrects << cand
end

ans = []

base.chars.each do |ch|
  t = (S + ch).chars.sort.join

  if corrects.include?(t)
    ans << ch
  end
end

if ans.empty?
  puts 'Impossible'
else
  puts ans
end