local str = io.read()
local moji = {"a","b","c","d","e","f","g","h","i","j","k","l","m"}
local cand = {}
for k, v in pairs(moji) do
  if(not string.find(str, v)) then table.insert(cand, v) end
end
if(#cand == 0) then
  for k, v in pairs(moji) do
    print(v)
  end
elseif(#cand == 1) then
  print(cand[1])
else
  print("Impossible")
end