import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

from collections import Counter

S = sorted(read().rstrip().decode())

se = set(S)
if len(se) != 7:
    print('Impossible')
    exit()

counter = Counter(S)
is_ok = True
for key,cnt in counter.items():
    if cnt not in [1,2]:
        is_ok = False
        break
    if cnt == 1:
        answer = key

if is_ok:
    print(answer)
else:
    print('Impossible')