import collections

s = input()

s_clc = collections.Counter(list(s))
one_count = 0
two_count = 0
target_char = None

for key in s_clc.keys():
    if s_clc[key] == 1:
        one_count += 1
        target_char = key
    elif s_clc[key] == 2:
        two_count += 1

res = None
if one_count == 1 and two_count == 6:
    res = target_char
else:
    res = 'Impossible'
    
print(res)