import collections

s=list(input())
c=collections.Counter(s)
a="abcdefghijklm"
if len(s)!=13:
    exit(print("Impossible"))

if len(c)==13:
    for x in list(a):
        print(x)
    exit()

elif len(c)==12:
    for x in a:
        if not x in c:
            print(x)
            exit()
else:
    print("Impossible")