S = list(input())
NOT_equal = []
for i in range(len(S)-1):
    if S.count(S[i]) == 3:
        print("Impossible")
        break
    if S.count(S[i]) == 1:
        NOT_equal.append(S[i])
else:
    if len(NOT_equal) != 1:
        print("Impossible")
    else:
        print(NOT_equal[0])