s = input()

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

x = max(c.values())

if x > 2:
    print('Impossible')
else:
    l = []
    for i in c:
        if x != c[i]:
            l.append(i)
    if len(l) >=2:
        print('Impossible')
    else:
        print(*l)