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)