from collections import defaultdict

S = input()

d = defaultdict(int)
for s in S:
    d[s] += 1

cnt = 0
res = 0
for k, v in d.items():
    if v == 2:
        cnt += 1
    if v == 1:
        res = k

if cnt == 6:
    print(res)
else:
    print("Impossible")