# -*- coding: utf-8 -*-

S = input()

hand = {}
yaochuhai = {'a','b','c','d','e','f','g','h','i','j','k','l','m'}

#国士無双素材
for hai in yaochuhai:
    hand[hai] = 0

#手牌確認
for hai in S:
    if hai in yaochuhai:
        hand[hai] += 1

miss = 0
miss_hai = ''
for hai in hand:
    if hand[hai] == 0:
        miss += 1
        miss_hai = hai

if miss >= 2:
    print('Impossible')
elif miss == 1:
    print(miss_hai)
else:
    for hai in yaochuhai:
        print(hai)