D = {s: 0 for s in "abcdefghijklm"}
S = input()
for s in S :
    D[s] += 1
V = list(D.values())
if (V.count(1) == 13) :
    for s in D :
        print(s)
elif (V.count(1) == 11 and V.count(2) == 1) :
    for s in D :
        if D[s] == 0 :
            print(s)
            break
else :
    print("Impossible")