def main():
    S = input()

    str_ = dict()
    for s in S:
        str_[s] = str_.get(s, 0) + 1

    cnt = 0
    for k, v in str_.items():
        if v == 1:
            cnt += 1
            ans = k
        elif v > 2:
            ans = 'Impossible'
            break
    else:
        if cnt > 1:
            ans = 'Impossible'

    print(ans)

main()