import sequtils, strutils
let s: string = stdin.readline
var
  a: array[26, int]
  ans: string = "Impossible"
for i in 0..<s.len:
  a[ord(s[i]) - ord('a')].inc
if a.count(2) == 6:
  ans = $(chr(a.find(1) + ord('a')))
echo ans