s = input()
dic = {}

for i in s:
    dic.setdefault(i,0)
    dic[i]+=1
    if dic[i]==3:
        print("Impossible")
        exit(0)

cnt = 0
ans = ""
for k,n in dic.items():
    if n==1:
        cnt+=1
        if cnt==1:
            ans=k
        else:
            print("Impossible")
            exit(0)
print(ans)