from collections import Counter
S = list(input())
S.sort()
C = Counter(S)
f = False
for k, v in C.items():
    if v == 1:
        if not f:
            f = True
            res = k
        else:
            print('Impossible')
            break
    elif v == 2:
        continue
    else:
        print('Impossible')
        break
else:
    if not f:
        print('Impossible')
    else:
        print(res)