read s; echo ${s} | fold -s1 | sort | uniq -c | sort -n | awk '{
  num = num $1
  chars[$2] = $1
} END {
  if (num == "111111111112") {
    split("abcdefghijklm", alphabets, "")
    for (i in alphabets) {
      if (!(alphabets[i] in chars)) print alphabets[i];
    }
  } else if (num == "1111111111111") {
    for (ch in chars) print ch;
  } else {
    print "Impossible"
  }
}' | sort