from collections import defaultdict

s = input()
dd = defaultdict(int)
for c in s:
    dd[c] += 1

a, b = 0, 0
c = ""
for d in dd:
    if dd[d] == 2:
        b += 1
    elif dd[d] == 1:
        a += 1
        c = d

print(c if (a, b) == (1, 6) else "Impossible")