S = input().strip()

dic = {}
for i in S:
  try:
    dic[i] += 1
  except:
    dic[i] = 1

if len(dic) != 7:
  print('Impossible')
  exit(0)

lst = [i for i in dic if dic[i] == 1]
if len(lst) > 1:
  print('Impossible')
  exit(0)

print(lst[0])