from collections import Counter

S=input()

C=Counter(S)

ANS=[]
for s in "abcdefghijklm":
    C[s]+=1

    two=0
    flag1=1

    for t in "abcdefghijklm":
        if C[t]==2:
            two+=1
        if 1<=C[t]<=2:
            pass
        else:
            flag1=0

    C[s]-=1

    if two==1 and flag1==1:
        ANS.append(s)

if len(ANS)==0:
    print("Impossible")
else:
    for ans in ANS:
        print(ans)