s = input()
strings = list("abcdefghijklm")
data = []
for i in strings:
    data.append(s.count(i))
if data.count(0) > 1:
    print("Impossible")
elif data.count(0) == 0:
    for i in strings:
        print(i)
else:
    j = data.index(0)
    print(strings[j])