#yujk_587
import sys
s=raw_input()

d={}
for c in s:
 if not c in d:
  d[c] = 1
 else:
  d[c] += 1

dk=d.keys()
dv=d.values()
if dv.count(1)!=1:
 print 'impossible'
 sys.exit()
if len(dv)!=7:
 print 'impossible'
 sys.exit()
for i in xrange(len(dv)):
 if dv[i]==1:
  print dk[i]