S = input()
cnt = [0]*26
for s in S:
    cnt[ord(s) - 97] += 1

cnt2 = 0
cnt1 = 0

for i in range(26):
    x = cnt[i]
    if x == 2:
        cnt2 += 1
    if x == 1:
        if cnt1 == 0:
            ans = i
            cnt1 += 1
        else:
            exit(print('Impossible'))

if cnt2 == 6:
    print(chr(97 + ans))
else:
    print('Impossible')