from itertools import groupby
s = input()
se = set(s)
freq = [len(list(group)) for key, group in groupby(s)]
f = {len(list(group)):key for key, group in groupby(s)}
num2 = len(list(filter(lambda x :x==2,freq)))
if num2==6 and len(s)==7:
    print(f[1])
else:
    print("Impossible")