S=input()
C="abcdefghijklm"
cnt=[]
for i in C:
    cnt.append(S.count(i))
if max(cnt)>2 or cnt.count(2)>=2:
    print("Impossible")
elif cnt.count(2)==1:
    for k in range(len(C)):
        if cnt[k]==0:
            print(C[k])
else:
    for k in range(len(C)):
        if cnt[k]==1:
            print(C[k])