with open ('24-157.txt') as f:
      s = f.readline()
def gip(s):
      return s[0] == s[2]
prin = filter(gip, zip (s, s[1:], s[2:]))
smotr = tuple(map(lambda a: a[1], prin))
bukv = sorted('QWERTYUIOPASDFGHJKLZXCVBNM')
give = tuple(map(lambda t: smotr.count(t), bukv))
print(bukv[give.index(max(give))] + str(max(give)))