from collections import Counter

s = input()
c = Counter(s)
cnt = 0
ans = ""

for i in c:
    if c[i] >= 3:
        print("Impossible")
        exit()

    elif c[i] == 1:
        ans = i
        cnt += 1

    if cnt > 1:
        print("Impossible")
        exit()

print(ans)