s = input()

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

if max(c.values()) <= 2:
    x = min(c.values())
    l = []
    for i in c:
        if x == c[i]:
            l.append(i)
            if len(l) < 2:
                print(*l)
else:
    print('Impossible')