sq = list(input())

dct = {}
for i in sq:
    dct.setdefault(i,0)
    dct[i] += 1

ls = list(dct.values())
ls.sort()
if ls == [1,2,2,2,2,2,2]:
    print([i for i,j in dct.items() if j == 1][0])
else:
    print('Impossible')