import sequtils, strutils, algorithm, math

var
  s: string = readLine(stdin)
  twoPair:int
  reach:char

for i in s:
  if s.count(i) == 2:
    twoPair += 1
  elif s.count(i) == 1:
    reach = i
  else:
    echo "Impossible"
    quit()

echo reach