from collections import Counter

S = input()

C = Counter(S)
ans = None
for k, v in C.items():
    if ans:
        if v != 2:
            print("Impossible")
            quit()
    else:
        if v == 1:
            ans = k
        elif v == 2:
            pass
        else:
            print("Impossible")
            quit()

print(ans)