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

import sys
input = sys.stdin.readline
l = int(input())
s = str(input())
occurenece = dict()
for i in range(0,len(s)-2):
  if not occurenece.__contains__(f'{s[i]}{s[i+1]}'):
    occurenece[f'{s[i]}{s[i+1]}'] = 1
  else:
    occurenece[f'{s[i]}{s[i+1]}'] = occurenece[f'{s[i]}{s[i+1]}'] + 1
max_value = 0
max_index = 0
values = list(occurenece.values())
keys = list(occurenece.keys())
for i in range(len(values)):
  if values[i]>max_value:
    max_value = values[i]
    max_index = i
print(keys[max_index])
  