from collections import Counter

S = input()
s = Counter(S)

num = 0


for i in s:
    if s[i] > 2:
        print("Impossible")
        exit()
    if num > 1:
        print("Impossible")
        exit()

    if s[i] == 2:
        num += 1

kokusi = "abcdefghijklm"
for i in range(len(kokusi)):
    if num == 1:
        if S.count(kokusi[i]) == 0:
            print(kokusi[i])

    else:
        if S.count(kokusi[i]) == 0 or S.count(kokusi[i]) == 1:
            print(kokusi[i])