S = gets.chop

t = nil

S.chars do |c|
  if c.ord < ?a.ord || ?m.ord < c.ord || S.count(c) > 2
    puts "Impossible"
    exit
  end
  if S.count(c) == 2
    next if t == c
    t ||= c
    puts "Impossible" if t != c
  end
end

if !t
  (?a..?m).each do |c|
    puts c
  end
else 
  (?a..?m).each do |c|
    puts c if S.count(c) == 0
  end
end