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 collections

def f(s, p):
    t = s[:]
    t += '*'
    c = 0
    for i in range(len(s)-2):
        if t[i:i+2] == p:
            c += 1
    return c

n = int(raw_input())
s = raw_input()
d = collections.defaultdict(int)
max_c = [0, '']
seen = set()
for i in range(0, len(s)-1):
    p = s[i: i+2]
    if p in d:
        continue
    seen.add(p)
    tmp = f(s, p)
    if tmp > max_c[0]:
        max_c[0] = tmp
        max_c[1] = p
if n == 2:
    max_c[1] = s
print max_c[1]
    

