15
MIRZOYANOVECLOX
 23
AABBBAAACCCCCAAADDDDDDD
 2
QA
 11
GGRRAATTZZZ
 6
AZAZAZ
 10
SQSQSQSQTG
 3
KEK
 100
URXCAIZFIBNJTPCZHBQIBCILLPXZCFGMKKZMNPLCYGAVJVIBMCZEBSJWPSCPQDYCTTKPOKIJRSKIZPDGCHVOUTMPNECYORSFZFNC
 3
LOL
 2
WW
 8
PUTINVOR
 26
QWERTYUIOPASDFGHJKLZXCVBNM
 5
AZAZA
 9
EGORLETOV
 7
KADUROV
 9
MIKEPIDOR
 9
AAAAAAAAA
 100
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
 5
FUFEL
 50
NYQAHBYYOXLTRYQDMVENEMAQNBAKGLGQOLXNAIFNQTOCLNNQIA

MI
 DD
 QA
 ZZ
 AZ
 SQ
 KE
 IB
 LO
 WW
 PU
 QW
 AZ
 EG
 KA
 MI
 AA
 AA
 FU
 YQ

def CountDict(Text, k):
  if k > len(Text): return None
  if k == len(Text): return {Text:1}
  list_substr = list()
  for j in range(len(Text)-k+1):
    list_substr.append(Text[j:j+k])
  RetMap = dict()
  for j in range(len(list_substr)):
    key_val = list_substr[j]
    RetMap[key_val] = RetMap.get(key_val,0)+1
  return RetMap

def PrintDict(d):
  v=list(d.values())
  k=list(d.keys())
  return k[v.index(max(v))]

n = int(input())
s = input()
answer = CountDict(s,2)
print(PrintDict(answer))   