s1 = "abcdefghijklm".split('')
s = gets.chomp.split('')
h = Hash.new(0)
s.each do |c|
    h[c] += 1
end
ans = true
flag1 = -1
flag2 = -1
s1.each do |c|
    if h[c] > 2
        ans = false
    elsif h[c] == 2
        if flag1 == -1
            flag1 = c
        else
            ans = false
        end
    elsif h[c] == 0
        if flag2 == -1
            flag2 = c
        else
            ans = false
        end
    end
end
if (flag1 != -1) ^ (flag2 != -1)
    ans = false
end
if !ans
    puts "Impossible"
    exit
end
if (flag1 != -1) && (flag2 != -1)
    puts flag2
else
    puts s1
end