S = input().strip()

target_chars = 'abcdefghijklm'
current_counts = {c: 0 for c in target_chars}
for c in S:
    current_counts[c] += 1

candidates = []

for char_to_add in target_chars:
    new_counts = current_counts.copy()
    new_counts[char_to_add] += 1

    valid = True
    count_of_twos = 0

    for c in target_chars:
        cnt = new_counts[c]
        if cnt < 1 or cnt >= 3:
            valid = False
            break
        if cnt == 2:
            count_of_twos += 1

    if valid and count_of_twos == 1:
        candidates.append(char_to_add)

if not candidates:
    print("Impossible")
else:
    print('\n'.join(sorted(candidates)))