sample, strs = "abcdefghijklm".split(""), gets.chomp.split("")
strs.each{ |val|
	if strs.count(val) == 1 
		sample.delete(val) 
	elsif !sample.include?(val) || strs.count(val)  > 2
		puts "Impossible"
		exit!
	end
}
sample.each{ |val| 
	if sample.count(val) == 1
		puts val
		exit!
	end
}
"a".upto("m"){ |val| puts val } if sample == []