import collections

S = input()
count_S = collections.Counter(S)
S_list = list(S)
S_set = set(S)

if len(S_list)-len(S_set)==6 and 3 not in count_S.values():
    for key,value in count_S.items():
        if value==1:
            print(key)
else:
    print("Impossible")