s = input()
d = {}
for c in s:
    if c in d:
        d[c] += 1
    else:
        d[c] = 1
m = []
for k, v in d.items:
    if v >= 3:
        print("Impossible")
        break
    elif v == 1:
        m.append(k)
else:
    if len(m) == 1:
        print(m)
    else:
        print("Impossible")