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

c = 0
for s in D :
    if D[s] % 2 == 1 :
        a = s
        c += 1

if c == 1 :
    print(a)
else :
    print("Impossible")