s=input()
from collections import defaultdict as df
dic=df(int)

for i in s:
    dic[i]+=1
    
vv=[]
kk=[]

for k,v in dic.items():
    if v != 2:
        vv.append(v)
        kk.append(k)
        
if len(vv)==1:
    if vv[0]==1:
        print(kk[0])
    else:
        print('Impossible')
else:
    print('Impossible')