def solve():
    s = input()
    d = dict()
    for c in s:
        d[c] = d.get(c, 0)+1
    key = None
    for k, v in d.items():
        if v > 2 or key and v == 1:
            print("Impossible")
            break
        if v == 1:
            key = k
    else:
        print(key)


if __name__ == "__main__":
    solve()