from collections import Counter


def main():
    S = input()
    hand_counter = Counter(S)
    hand_comb = hand_counter.most_common(7)
    if hand_comb[0][1] == 2 and hand_comb[-1][1] == 1:
        print(hand_comb[-1][0])
    else:
        print("Impossible")


if __name__ == "__main__":
    main()