import sys

sys.setrecursionlimit(10 ** 6)
stdin = sys.stdin

ni = lambda: int(ns())
na = lambda: list(map(int, stdin.readline().split()))
ns = lambda: stdin.readline().strip()
ntp = lambda: tuple(map(int, stdin.readline().split()))
mod = 10 * 9 + 7
inf = 2 << 60

S = list(ns())
T = set(S)
ans = '-1'
for t in T:
    cnt = S.count(t)
    if cnt == 1:
        if ans == '-1':
            ans = t
        else:
            print('Impossible')
            exit()
    if cnt > 2:
        print('Impossible')
        exit()

print(ans)