import sys
input=lambda: sys.stdin.readline().rstrip()
A=["a","b","c","d","e","f","g","h","i","j","k","l","m"]
S=input()
for s in S:
  while s in A:
    A.remove(s)
if not A:
  print("a")
elif len(A)==1:
  print(*A)
else:
  print("Impossible")