S = list(input())
S.sort()

l = len(S)

i = 0
count = -1
while True:
	if i >= l - 1:
		break

	p = S.count(S[i])

	if p > 2:
		print("Impossible")
		exit(0)

	elif p == 1: 
		if count != (-1):
			print("Impossible")
			exit(0)
		else:
			count = i
			i += 1
	
	else:
		i += 2

print(S[count])