a = [*"abcdefghijklm"]
kouho = set()
for ai in a:
    kouho.add(tuple(sorted(a+[ai])))
s = [*input()]
ok = 0
for ai in a:
    if tuple(sorted(s+[ai])) in kouho:
        ok = 1
        print(ai)
if not ok:
    print("Impossible")