from string import ascii_lowercase
D = {s : 0 for s in ascii_lowercase}
for s in input() :
    D[s] += 1

flg = False
for s in D :
    if D[s] > 2 :
        print("Impossible")
        break
    elif D[s] == 1 :
        if not flg :
            a = s
            flg = True
        else :
            print("Impossible")
            break
else :
    print(a)