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 re

n=int(input())
s=input()
ted={}
for i in range(1,n):
	if s[i-1:i+1] in ted:
		ted[s[i-1:i+1]]+=1
	else:
		ted[s[i-1:i+1]]=1
mx=max(ted.values())
for key,val in ted.items():
	if val==mx:
		print(key)
		break
# ans=''
# for c1 in range(ord('A'),ord('Z')+1):
# 	for c2 in range(ord('A'),ord('Z')+1):
# 		cc=chr(c1)+chr(c2)
# 		print(cc,s.count(cc))
# 		if mx<len(re.findall(cc,s)):
# 			mx=len(re.findall(cc,s))
# 			ans=cc
# print(ans)