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

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

full = set('abcdefghijklm')
se = full - set(S)

if len(se) >= 2:
    answer = 'Impossible'
elif len(se) == 1:
    answer = se.pop()
else:
    answer = '\n'.join(sorted(full))

print(answer)