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


# Problem: B. Two-gram
# Contest: Codeforces - Codeforces Round #479 (Div. 3)
# URL: https://codeforces.com/problemset/problem/977/B
# Memory Limit: 256 MB
# Time Limit: 1000 ms
# Powered by CP Editor (https://github.com/cpeditor/cpeditor)

n=int(input())
S=input()
D=dict()
for _ in range(n-1):
	if S[_]+S[_+1] in D:
		D[S[_]+S[_+1]]+=1
	else:
		D[S[_]+S[_+1]]=1
m=0
for _ in D:
	if D[_]>m:
		u=_
		m=D[_]
print(u)	