S = input()
from collections import defaultdict
dic = defaultdict(int)

for s in S:
    dic[s] += 1

p1 = 0
p2 = 0
ans = ""
for key, value in dic.items():
    if value == 2:
        p2 += 1
    elif value == 1:
        p1 += 1
        ans = key
    else:
        print("Impossible")
        break
else:
    if p1 == 1:
        print(ans)
    else:
        print("Impossible")