import collections

s=str(input())
count=0
ans=str()
for i in range(len(s)):
    if s.count(s[i])!=2:
        if s.count(s[i])!=1:
            print("Impossible")
            exit()
        elif s.count(s[i])==1:
           count+=1
           ans=s[i]

if count==1:
    print(ans)
else:
    print("Impossible")