import tables
let S = stdin.readLine
var
  dic = initCountTable[char]()
  flg: bool
for s in S:
  dic.inc s

for a in ('a' .. 'm'):
  var dic2 = dic
  dic2.inc a
  if dic2.len >= 13 and dic2.largest.val >= 2:
    echo a
    flg = true

if not flg: echo "Impossible"