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

l = len(S)

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

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

ans = S[count]

S.remove(ans)


if ans in S:
	print("Impossible")
else:
	print(ans)