def ascan; gets.split.map(&:to_i);end

def bye(msg)
    puts msg
    exit
end

s = gets.chomp

mem = {}
s.chars.each do |c|
    mem[c] ||= 0
    mem[c] += 1
end

one = nil

mem.each do |k,v|
    bye "Impossible" if v == 1 && !one.nil?
    one = k if v == 1 && one.nil?
    bye "Impossible" if v >= 3
end

abort if one.nil?

puts one