s = gets.chomp
h = Hash.new(0)
s.scan(/./){|i| h[i] += 1}
j = 0
m = 0
h.each do |k,v|
  if v > 2
    j += 2
  elsif v == 1
    j += 1
    m = k
  end
end
if j == 1
  puts m
elsif
  puts "Impossible"
end