S = input()
_set = set(S)
can_pair = len(_set) == 7
ans = "Impossible"
if can_pair:
    chars = {}
    for s in _set:
        cnt = S.count(s)
        if cnt > 2:
            break
        chars[s] = cnt
    else:
        ans = min(chars.items(),key=lambda x:x[1])[0]
print(ans)