s = str(input())
X = [0]*13
for i in range(len(s)):
    X[ord(s[i])-ord('a')] += 1

import copy
Y = copy.copy(X)
Y.sort()
p1 = [0]+[1]*11+[2]
p2 = [1]*13
if Y == p1:
    for i in range(13):
        if X[i] == 0:
            print(chr(i+ord('a')))
            exit()
if Y == p2:
    for i in range(13):
        print(chr(i+ord('a')))
    exit()

print('Impossible')