s = input()

d = {}
for i in s:
    if not i in d:
        d[i] = 1
    else:
        d[i] += 1

l = []
for k, v in d.items():
    if v == 1:
        l.append(k)

if len(l) == 1:
    print(*l)
else:
    print('Impossible')