#yujk_587

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()
l=[]
for i in xrange(len(dv)):
 if dv[i]!=2:
  l.append(dk[i])
if len(l)!=1:
 print 'impossible'
else:
 print l[0]