s,dic,v=input(),{},[]
for c in s:
    if c not in dic:
        dic[c]=1
    else:
        dic[c]+=1
for e in dic.values():
    v.append(e)
v.sort()
if v != [1,2,2,2,2,2,2]:
    print("Impossible")
else:
    for e in dic.keys():
        if dic[e]==1:
            print(e)
            break