s = gets.chomp.split("")
s.sort!
s << 1
m = 0
for i in 0..7
  if s[0] != s[1]
    puts s[0]
    break
  elsif s[2*i] == s[2*i+1] && s[2*i+1] == s[2*i+2]
    puts "Impossible"
    break
  elsif s[2*i] != s[2*i+1]
    if i == 6
      puts s[12]
      break
    else
      s[2*i+2] == s[2*i+1] ? m = s[2*i] : m = s[2*i+1]
      puts m
      break
    end
  end
end