s=input()
cnt=[0]*26
for c in s:
    cnt[ord(c)-ord('a')]+=1
one=[]
two=0
for i in range(26):
    if cnt[i]==1:
        one.append(chr(ord('a')+i))
    elif cnt[i]==2:
        two+=1
if two==6 and len(one)==1:
    print(one[0])
else:
    print('Impossible')