s = input()

def find(s):
  p = []
  for i in s:
    if len(p) >= 2:
      return 'Impossible'

    if s.count(i) == 1:
      p.append(i)
    
    if s.count(i) >= 3:
      return 'Impossible'

  if len(p) == 1:
    return p[0]

print(find(s))