k='abcdefghijklm'
v=['Impossible']
s=input()
l=len(set(s))
if sum([i in k for i in s])==13:
  if l==13:
    v=[i for i in k]
  elif l==12:
    for i in k:
      if i not in s:
        v=[i]
        break
for i in v:print(i)