from collections import defaultdict
dic = defaultdict(int)
s = list(input())
for k in s:
    dic[k] += 1

num, ans = 0, ""
if max(dic.values()) >= 3:
    num = 3
else:
    for k, v in dic.items():
        if v == 1:
            num += 1
            ans = k

if num == 1:
    print(ans)
else:
    print("Impossible")