s = list(str(input()))
from collections import Counter
C = Counter(s)
D = list(C.items())
D.sort(key=lambda x: x[1])
if len(D) != 7:
    print('Impossible')
    exit()

for i in range(7):
    k, v = D[i]
    if i == 0:
        if v != 1:
            print('Impossible')
            exit()
        else:
            ans = k
    else:
        if v != 2:
            print('Impossible')
            exit()
else:
    print(ans)