def main():
	s = input()
	s = sorted(s)
	c=""
	for i in range(int(len(s)/2)):
		if s[0] == s[1]:
			del s[0:2]
		elif (c==""):
			c=s[0]
		else:
			print("Impossible\n")
			return
	if (c!=""):
		print(c)
	else:
		print(s[-1])

main()