from collections import Counter

st=input()

cnt=Counter(st)

kels=[]
ngflg=0

for key,value in cnt.items():
        if value==1:
                kels.append(key)      
        if value != 1 and value != 2:
                ngflg=1

if len(kels)==1 and ngflg==0:
        print(kels[0])
else:
        print("Impossible")