with open ('24j.txt') as f:
    s = f.readline()
f_c = sorted(map(lambda x:x[2], filter(lambda t: t[0] == t[1], zip(s, s[1:], s[2:]))))
f1 = sorted(set(f_c))
kav = list(map(lambda x : f_c.count(x), f1))
vak = max(kav)
print(f1[kav.index(vak)])