s=gets.chomp
a=s.split('')
b=a.uniq
if b.size != 7
    puts "Impossible"
else
    hoge = b.map{|c|s.count(c)}
    if hoge.sort != [1,2,2,2,2,2,2]
        puts "Impossible"
    else
        puts b[hoge.index(1)]
    end
end